A CO-TWIN CONTROL DESIGN EXAMINING SOCIALIZATION VERSUS GENETIC SELECTION EFFECTS IN THE ASSOCIATION BETWEEN BODY-CONSCIOUS PEER GROUPS AND DISORDERED EATING By Shannon M. O’Connor A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Psychology-Master of Arts 2014 ABSTRACT WHAT DRIVES THE ASSOCIATION BETWEEN BODY-CONSCIOUS PEER GROUPS AND DISORDERED EATING? DISENTANGLING GENETIC AND ENVIRONMENTAL SELECTION FROM PURE SOCIALIZATION EFFECTS By Shannon M. O’Connor Previous studies suggest strong associations between body-conscious peer groups and disordered eating. This association has been attributed to socialization effects (i.e., membership in peer groups leads to disordered eating); however, selection effects (i.e., selecting into peers group based on genetic or environmental predispositions toward disordered eating) could contribute to or even account for these associations. The current study was the first to use a cotwin control design to disentangle genetic and shared environmental selection factors from pure socialization effects. Participants included 432 female twins (ages 8-14) drawn from the Michigan State University Twin Registry. To comprehensively examine the full range of eating pathology, several disordered eating attitudes and behaviors (e.g., body dissatisfaction, binge eating) were examined via self-report. Self-report questionnaires also were used to assess peer group emphasis on body weight and shape. Replicating previous results, significant withinperson associations were found between membership in body-conscious peer groups and disordered eating. However, co-twin control analyses indicated that this association was due entirely to genetic and shared environmental selection factors. Results remained unchanged when controlling for age, BMI, and pubertal status. These findings question the current predominate theory that the association between body-conscious peer groups and disordered eating is due entirely to socialization processes. Future studies are needed to identify the specific genetic and shared environmental factors that may drive selection into body-conscious peer groups. TABLE OF CONTENTS LIST OF TABLES..........................................................................................................................iv LIST OF FIGURES.........................................................................................................................v Introduction......................................................................................................................................1 Methods............................................................................................................................................8 Participants...........................................................................................................................8 Zygosity Determination...........................................................................................9 Measures..............................................................................................................................9 Body Conscious Friend Groups...............................................................................9 Disordered Eating..................................................................................................11 General Disordered Eating Attitudes and Behaviors.................................11 Loss of Control Over Eating......................................................................13 Pubertal Development............................................................................................14 BMI .......................................................................................................................15 Data Analyses....................................................................................................................15 Data Preparation.....................................................................................................15 Initial Associations.................................................................................................16 Co-twin Control Analyses......................................................................................16 Results............................................................................................................................................21 Scale Intercorrelations and Informant Report....................................................................21 Initial Phenotypic Associations Between Disordered Eating and Exposure to Body Conscious Peer Groups......................................................................................................21 Co-twin Control Analysis Using Twin Report of Disordered Eating Symptoms..............22 Co-twin Control Analyses Using Parental Report of Twin’s Disordered Eating Symptoms..........................................................................................................................23 Discussion......................................................................................................................................25 APPENDICES...............................................................................................................................33 APPENDIX A: Tables.......................................................................................................34 APPENDIX B: Figures......................................................................................................48 REFERENCES..............................................................................................................................50 iii LIST OF TABLES Table 1 Intercorrelations between Scores on the Body Conscious Peer Group Scales (N = 383388)................................................................................................................................................35 Table 2 Pearson Correlations examining Associations between Twin, Maternal and Paternal Reports of Disordered Eating (Twin Report N=406-430; Maternal Report N=401-420; Paternal Report N= 140-142) ......................................................................................................................36 Table 3 Pearson Correlations examining Within-Person Associations between Body Conscious Peer Groups and Disordered Eating in the Full Sample using Twin Self Report..........................37 Table 4 Results from Co-Twin Control Analyses examining Associations between BodyConscious Peer Groups and Disordered Eating using Twin Self-Report..................................... 38 Table 5 Results from Co-Twin Control Analyses Examining Associations between BodyConscious Peer Groups and Disordered Eating using Twin Self-Report Controlling for Age, BMI and Pubertal Status.........................................................................................................................40 Table 6 Results from Co-Twin Control Analyses examining Associations between BodyConscious Peer Groups and Disordered Eating using Twin Self-Report Controlling for Age, BMI and Maternal Disordered Eating....................................................................................................42 Table 7 Results from Co-Twin Control Analyses examining Associations between BodyConscious Peer Groups and Disordered Eating Using Maternal Report of Twin’s Disordered Eating Symptoms...........................................................................................................................44 Table 8 Results from Co-Twin Control Analyses examining Associations between BodyConscious Peer Groups and Disordered Eating Using Paternal Report of Twin’s Disordered Eating symptoms ...........................................................................................................................46 iv LIST OF FIGURES Figure 1 Summary of Potential Results of Co-Twin Control Analysis…………………………49 v Introduction Prior research has demonstrated a significant association between body-conscious or weight-focused peer groups and levels of disordered eating. Multiple cross-sectional studies have shown associations between peer groups’ emphasis on body weight and excessive weight concerns, dieting and body-image dissatisfaction in girls (Wertheim, Paxton, Schutz, & Muir, 1997; Taylor et al., 1998; Vander Wal & Thelen, 2000; Levine & Smolak, 1992). Likewise, body image concerns, dietary restraint, and the use of extreme weight-loss behaviors have been shown to be more similar within- than between-peer groups (Paxton, Schutz, Wertheim, & Muir, 1999). Longitudinal studies have corroborated cross-sectional effects by demonstrating prospective associations between peer groups’ level of weight concerns and disordered eating in girls across 6 month to 5 years (Haines, Neumark-Sztainer, Eisenberg, & Hannan, 2006; Myer and Waller, 2001; Crandall, 1988). Despite the significant association between body-conscious peer groups and levels of disordered eating, prior research has not adequately examined the underlying mechanisms in this association. Researchers have proposed mainly causal relationships between peer groups and disordered eating (Crandall, 1988), such that being a part of a body-conscious peer group leads to higher rates of disordered eating. Theories used to explain these causal relationships include social identity theory (Tajfel & Turner, 1979), which states that individuals who form a group will tend to adopt similar attitudes and behaviors in order to accentuate in-group similarities and out-group differences (Myer & Waller, 2001), and social comparison theory (Festinger, 1954), which proposes that individuals evaluate their own abilities and opinions by comparing themselves to others in their peer groups (Heinberg, 1996). 1 Two studies have indirectly examined these causal, social explanations by exploring the convergence and divergence of disordered eating within peer groups. Overall, findings show that peer groups that spend more time together become more similar in their eating behaviors than peer groups that spend less time together. For example, studying two separate sororities, Crandall (1988) found that binge eating levels were more similar in friendship groups at the end than at the beginning of the academic year; at the beginning of the year, a woman’s binge eating level was equally similar to her sorority friends as to the sorority as a whole, whereas at the end of the year, a woman’s level of binge eating was significantly more correlated with members of her sorority friendship group than non-members of her friendship group (Crandall, 1988). Examining divergence rather than convergence, Zalta & Keel (2006) also found that roommates who cohabited during the academic year became less similar to each other in bulimic symptoms (i.e., the EDI bulimia subscale; Garner, 1991) over their summer break away, suggesting that the lack of exposure to one’s roommates resulted in decreased similarity in bulimic symptoms between roommates. Despite evidence for socialization mechanisms, there are studies suggesting that selection effects (see Burt et al., 2010; McGue, Osler & Christesen, 2010) may also play a role in peer group influences on disordered eating. Selection effects occur when individuals seek out peer groups that are in line with their own attitudes and beliefs. In the case of disordered eating, girls who strongly value thin body weights and shapes might seek out peer groups who also focus on these characteristics. These weight-focused peers groups may then reinforce and strengthen their underlying beliefs and lead to increased disordered eating within the group. Although one study failed to find peer selection effects for disordered eating (i.e., women who selected their roommates were not more similar in their levels of drive for thinness, body 2 dissatisfaction, or bulimic symptoms than women who were unselected, i.e., randomly paired) (Gilbert & Myer, 2004), another study suggested that selection effects may be important. Myer and Waller (2001) examined convergence/divergence of bulimic symptoms in unselected college roommates to determine if socialization effects are present in new, unselected peer groups. Across three time points (i.e., 1 week after move in, and then 10 and 24 weeks later), unselected roommates did not become more similar to each other in bulimic symptoms; in fact, they became more dissimilar, as evidenced by a divergence in bulimic symptoms over the course of the study (Myer & Waller, 2001). These findings question the role of pure socialization processes in peer similarity for disordered eating and further suggest that socialization may be dependent upon an individual’s selection into a like-minded peer group, as socialization effects were not present when peers were completely unselected. Previous studies were unable to rule-out this latter possibility, as neither Crandall (1988) nor Zalta & Keel (2006) assessed whether sorority sisters or roommates were selected or unselected, i.e., it is unclear if subjects were friends prior to joining the sorority or becoming roommates. It may be that the socialization effects observed in these studies were predicated on initial selection into like-minded peer groups that was then reinforced by exposure to weight-focused peers. Clearly, additional research is needed to determine the causal role of weight-focused peers on eating disorder risk and differentiate socialization from selection effects. One approach for investigating these processes is the co-twin control study. The co-twin control design is based on the counter-factual model (McGue et al., 2010), which allows for the identification of causal influences and differentiation between socialization and selection effects. The basis of the counterfactual model is that the best way to determine whether a risk factor (e.g., exposure to weight-focused peers) is causal for a particular outcome (versus non-causal or due to selection 3 processes) is to examine the outcome when the individual is exposed to the risk factor and when the individual is not exposed. If the outcome is the same regardless of whether the individual is exposed or not-exposed, then the risk factor is not causal, i.e., it does not affect the outcome. However, if the individual’s outcome is different depending on whether he/she was exposed, then it is likely that the risk factor is causal for the outcome. Unfortunately, it is impossible to observe an outcome for both exposure and non-exposure simultaneously in one person. Consequently, the counterfactual model attempts to get as close as possible by matching individuals in the exposed versus non-exposed groups on key characteristics. This matching ensures that exposure to the risk factor is not based on selection via pre-existing traits, as the two groups are matched on any traits that could lead to selection into the exposed group. The matching also allows one to estimate the non-exposed outcome for exposed individuals, and conversely, the exposed outcome for non-exposed individuals (Burt et al., 2010). The co-twin control method extends the counterfactual model by examining co-twins discordant on exposure to the risk factor (e.g., body-conscious peer groups) to predict each twin’s level on the outcome (i.e., disordered eating symptoms). This method eliminates the need for matching since twins are already matched on key demographic characteristics (i.e., age, socioeconomic status, etc.). Moreover, the model improves upon the typical counterfactual model by matching twins on shared environmental influences (i.e., environmental influences shared by siblings, e.g., parental divorce, parental discipline, etc.) and genetic predispositions, as twins reared together share 100% of their shared environment, and ~50% (for dizygotic (DZ) twins) or 100% (for monozygotic (MZ) twins) of their genes. The ability to control for and model genetic risk is particularly important for disordered eating, as twin and adoption studies show significant genetic influences (i.e., heritability ≥ 50%) on clinical eating disorders and their 4 symptoms (Bulik, Sullivan & Kendler, 1998; Klump, Burt, Spanos, McGue, Iacono, & Wade, 2010; Kaye, Klump, Frank, & Strober., 2000; Klump, Miller, Keel, McGue, & Iacono, 2001). Genetic risk for disordered eating could contribute to selection of body-focused peer groups, as young girls who have a genetic predisposition for disordered eating may be more likely to join friendships groups that mirror their own emphasis on body weight and/or shape. Capitalizing on the use of twins, the co-twin control study compares three different sets of regression results to differentiate causal, socialization processes from selection effects: (1) individual-level effects (i.e., the extent to which a twin’s exposure to weight focused peers predicts her own level of disordered eating), (2) effects in DZ twin pairs only (i.e., the extent to which within-pair discordance on exposure to weight focused peers predicts each DZ twin’s level of disordered eating), and (3) effects in MZ twins only (i.e., the extent to which within-pair discordance on exposure to weight focused peers predicts each MZ twin’s level of disordered eating). The presence of significant individual-level effects would be similar to what has been shown in previous cross-sectional research - they simply show significant within-person associations, not differentiating socialization from selection effects. By contrast, discordant MZ and DZ twin pair effects help determine if within-person associations are due to socialization or selection effects. Because MZ and DZ co-twins share 100% of their shared environment, discordance between them on the exposure variable cannot be due to shared environmental selection effects. Thus, if shared environmental factors are important for selection into weight-focused peer groups, significant associations between exposure and disordered eating in either discordant MZ or DZ twins will not be observed. By contrast, if genetic factors are important for selection into weight-focused peer groups, then significant associations between exposure to weight-focused peers and disordered eating in 5 discordant DZ twins will be observed, since these twins only share 50% of their genes. By contrast, an association in discordant MZ twin pairs would not be observed, since these twins share 100% of their genes – in this case, discordance in exposure to weight-focused peers cannot be due to differences in genetic factors between MZ co-twins. Figure 1 provides a summary of these expected results if socialization and/or selection effects are present. Scenario A shows expected results if associations between weight-focused peers and disordered eating are due entirely to socialization processes. In this case, there are significant associations within individual and within discordant MZ and DZ twin pairs, suggesting that exposure to weight-focused peers increases disordered eating within-person and within-twin pairs even when shared environmental and genetic selection processes are controlled. Scenarios B and C show expected results if full or partial selection is present. In both cases, the association between exposure to body conscious friend groups and disordered eating in discordant twins is attenuated, suggesting that the association is due to either genetic and/or shared environment selection effects. Scenario B would suggest genetic selection effects only, as there is still an association for DZ twins who share less genetic material than MZ twins. Scenario C would suggest both genetic and shared environmental selection effects, since significant associations are not present when not controlling for shared environmental and genetic selection processes. Notably, the co-twin control method is unable to control for all environmental risk factors, in particular, non-shared environmental influences. Non-shared environmental factors include environmental experiences that are unique to co-twins raised in the same family (e.g., a dance instructor encourages one MZ co-twin to continue with dance but not the other). These non-shared experiences could lead to selection into the exposure group (e.g., weight-focused 6 peers in ballet class). Thus, even if selection does not appear to be present (Scenario A, Figure 1), it is possible that non-shared environmental factors led to selection into body-focused peer groups and are driving the within-person and discordant twin pair associations. Despite this shortcoming, however, the ability of the co-twin control method to rule-out genetic and shared environmental selection processes is still a significant step forward in determining a causal role for weight-focused peers in disordered eating risk. The present study improved upon past research by exploring selection factors versus pure socialization effects in the association between body conscious peer groups and disordered eating using the co-twin control design. In order to comprehensively examine the full range of eating pathology, this study investigated several disordered eating attitudes and behaviors (e.g., body dissatisfaction, weight preoccupation, etc.) that significantly predict the eventual development of clinical eating disorders (Stice & Shaw, 2002; Killen et al., 1996). The current study examined pre-adolescent and young adolescent girls, as prior research has focused almost exclusively on college-aged women with a lack of attention paid to the adolescent period when eating disorders begin and peer groups are critical for development. This study was the first to explore the possibility of genetic and environmental selection effects, providing a unique opportunity to disentangle the causal influences on this association. 7 Methods Participants This study used archival twin data (N=432 female twins ages 8-14 years, M=11.23; MZ N=222, 51%; DZ N=210, 49%) drawn from the Twin Study of Mood, Behavior, and Hormones during Puberty from the Michigan State University Twin Registry (MSUTR; Klump & Burt, 2006; Burt & Klump, 2012). Twins in the current study were recruited through another MSUTR study, the Michigan Twins Project (MTP). The MTP is a population-based registry of all twins born in Michigan ages 3-25 and 35-45. The MTP recruits twins through birth records in collaboration with the Michigan Department of Community Health (MDCH). Response rates for participation in the overall MTP is 58%, while response rates for the Twin Study of Mood, Behavior, and Hormones during Puberty were even higher at 73%. Both response rates are on par or better than those of other twin registries using similar recruitment methods (Burt & Klump, 2012; Iacono & McGue, 2002). The MSUTR and MTP are representative of the Michigan population with approximately 85% of participants identifying as white (non-Hispanic) and 15% of participants as non-white (Burt & Klump, 2012; Culbert, Breedlove, Burt & Klump, 2008; Michigan State Department of Technology, Management and Budget, 2010). Characteristics of the sample are similar to that of the MSUTR and MTP. The majority of participants are Caucasian (82.5%), although significant minorities are African American (9.5%) or indicated multiple races (7.5%). Notably, the majority of participants completed the study procedures in the laboratory at Michigan State University (63%), however, study procedures are also completed in the participant’s home by trained research assistants (37%). 8 Zygosity Determination The MSUTR determines zygosity using a physical similarity questionnaire (Lykken, Bouchard Jr, McGue, & Tellegen, 1990; Peeters, Van Gestel, Vlietinck, C. Derom, & R. Derom, 1998). For the sample used in this study, both twins, their mother and two research assistants evaluated the physical similarities independently. Reports are compared and any discrepancies are resolved through review of questionnaire data and twin photographs by one of the principal investigators or by examination of DNA markers (Klump & Burt, 2006). The use of the Physical Similarity Questionnaire is reported to have 95% accuracy or better when compared to genotyping (Peeters et al., 1998). Measures All of the following questionnaires were administered to each twin. For the disordered eating questionnaires, at least one parent (for ~97% of the sample, the mother) provided a parental report as well. However, for a subset of participants (~36% of the sample) both maternal and paternal reports were available. Body Conscious Friend Groups: The Perceived Friend Preoccupation with Weight and Dieting Scale (Schutz, Paxton, & Wertheim. 2002) is a 9-item questionnaire aimed at assessing twins’ perceptions of the frequency of weight- and dieting- related thoughts and behaviors among their friends. This self-report questionnaire has participants rate their response on a 5-point scale from 1 (never) to 5 (very often), thus high scores represent high perceived importance of weight and dieting among their friendship groups. Factor analysis found all items load on a single factor with item-total correlations ≥.52. Cronbach’s alpha in a sample of adolescent girls (mean age = 14.10 years) was excellent at .87 (Schutz et al., 2002). 9 The Appearance Conversations with Friends (Jones, Vigfusdottir, & Lee. 2004) is a 5item questionnaire modified from the Magazines as a Source of Influence Scale (Levine, Smolak, & Hayden, 1994) that is aimed at assessing how often adolescents discussed with friends their current and desired body shape (e.g., ”My friends and I talk about how our bodies look in our clothes”, “My friends and I talk about what we would like our bodies to look like”). Cronbach’s alpha of .85 indicated excellent reliability for a sample of adolescent females (mean age = 13.47) (Jones et al., 2004) The Friends as Source of Influence Scale (Paxton et al. 1999) is a 5-item questionnaire that asks participants to rate on a 5-point scale from 1 (Not at all important) to 5 (Very important) how important their friends are in influencing their opinions of the perfect body, diet products, exercise, and dieting (e.g. “Your idea of the perfect body”, “The diet products you use”). Cronbach’s alpha of .87 indicates good internal consistency in a study of 10th grade girls (mean age = 15.5) (Paxton et al., 1999). The Peer Attribution Scale (Lieberman, Gauvin, Bukowski, & White, 2001) originally included 8-items assessing appearance-related attributions from same- and opposite-sex peers. In the current sample, this questionnaire was cut to 4 items that focus on attributions from either sex peers (e.g., “My friends would like me more if I lost weight”); items relating to opposite-sex peers (e.g., “If I was thinner, boys would be more attracted to me”) were cut from the larger project to decrease subject burden. Subjects rate items on a 6-point Likert scale ranging from “false” to “true”. The original scale had a Cronbach’s alpha of .90 (Liebermann et al., 2001) and the modified scale exhibits a Cronbach’s alpha of .85 (Shroff &Thompson, 2006). 10 Disordered Eating: While the prevalence of clinical eating disorders is relatively rare within the general population (approximately .5% for anorexia nervosa and approximately 1-3% for bulimia nervosa; American Psychiatric Association, 2000), the prevalence of disordered eating attitudes and behaviors that contribute to the eventual development of clinical disorders (e.g., body dissatisfaction, weight preoccupation, binge eating, etc.) is more common (Favaro, Ferrara, & Santonastaso, 2003; Mintz & Betz, 1988) The current study therefore investigated a variety of disordered eating attitudes and behaviors (rather than clinical disorders) in order to maximize power and examine the full spectrum of eating pathology. The current study examined both general symptoms of clinical eating disorder (e.g., body dissatisfaction, weight preoccupation, binge eating) as well as placed an emphasis on loss of control over eating. Loss of control over eating has emerged as a strong early predictor of binge eating and disorders characterized by binge eating (i.e., bulimia nervosa and binge eating disorder), as it is often present in younger children prior to the development binge eating (Tanofsky-Kraff, Marcus, Yanovski & Yanovski, 2008a). Given the age range examined in this study, it was critical to examine loss of control over eating, particularly since previous studies of peer group effects have focused almost exclusively on eating disorders characterized by binge eating (Crandall et al., 1988; Gilbert & Myer, 2004; Zalta & Keel, 2006). General Disordered Eating Attitudes and Behaviors The Minnesota Eating Behavior Survey (MEBS; von Ranson, Klump, Iacono & McGue, 2005) is a 30-item questionnaire made up of true/false questions that aims to assess a spectrum of eating pathology on a continuum of severity. This measure was developed for use with children as young as 10-years-old. Factor analysis produced four factors: Body Dissatisfaction (i.e., 11 assessing discontent with body size and shape), Compensatory Behaviors (i.e., assessing the use of, and thoughts of using, self-induced vomiting and other inappropriate compensatory behaviors to control weight), Binge Eating (i.e., assessing thinking about binge eating as well as engaging in binge eating and/or secretive eating) and Weight Preoccupation (i.e., assessing preoccupation with weight, eating, and dieting) (von Ranson et al., 2005; Klump, McGue, & Iacono, 2000). The current study examined the Body Dissatisfaction, Binge Eating and Weight Preoccupation subscales, as well as the MEBS Total Score. The Compensatory Behaviors subscale was not included due to anticipated low variability of responses. Internal consistency for the Total Score, Weight Preoccupation subscale, Body Dissatisfaction subscale, and Binge Eating subscale in previous studies have ranged from alphas of 0.70-0.85 (von Ranson et al., 2005). Overall, the MEBS shows good three-year stability (0.32-0.59) with the Total score being the most stable followed by the scales measuring attitudes (Weight Preoccupation and Body Dissatisfaction) and then behavior (Binge Eating) (von Ranson et al., 2005). The MEBS shows good convergent validity with the Eating Disorder Examination Questionnaire (EDE-Q; Fairburns & Beglin, 1994). The strongest correlation was between the MEBS Total Score and the EDE-Q Total Score (r = 0.83), however high correlations (r=0.74-0.78) were also found between other subscales as well, including MEBS Weight Preoccupation with EDEQ Weight Concern (r=.78) and MEBS Body Dissatisfaction with EDEQ Shape Concerns (r=.74) (von Ranson et al., 2005). The MEBS also shows good criterion-related validity, as demonstrated by girls with eating disorders (i.e., either anorexia nervosa or bulimia nervosa) having significantly higher scores on the Body Dissatisfaction subscale, Weight Preoccupation subscale, and Total Score on the MEBS than controls (von Ranson et al., 2005). Additionally, participants with bulimia 12 nervosa had significantly higher scores on the Binge Eating subscale than controls (von Ranson et al. 2005). Loss of Control Over Eating The Eating in the Absence of Hunger for Children and Adolescents (EAH-Child; Tanosky-Kraff et al., 2008b) is a 14-item questionnaire that assesses the frequency of precipitants to eating when one is not hungry in 6-19-year old youth. This questionnaire explores 5 emotional precipitants to eating when one is not hungry (i.e., feeling sad or depressed, angry or frustrated, anxious or nervous, tired, bored) and 2 external precipitants (i.e., sensory cues, social cues). Using a 5-point Likert scale with answers ranging from 1 (“Never”) to 5 (“Always”), participants are asked to select the frequency in which they eat past satiation and in the absence of hunger in response to each of the 7 precipitants. Factor analysis generated three subscales: Negative Affect (i.e., eating in the absence of hunger in response to feeling sad or depressed, angry or frustrated, or anxious or nervous), External Eating (i.e., eating in absence of hunger when food looks, tastes, or smells good and when others are eating), and Fatigue/Boredom (i.e., eating in absence of hunger when feeling tired or bored) (Tanosky-Kraff et al., 2008b). Internal consistency was excellent for all subscales with Cronbach’s alphas ranging from 0.80-0.88 (Tanosky-Kraff, et al., 2008b). EAH-C shows good convergent validity with participants who endorsed loss of control when interviewed having significantly higher scores on all three subscales than participants who did not endorse loss of control (Tanosky-Kraff, et al., 2008b). Temporal stability was assessed by re-administration of the questionnaire 5-565 days after initial assessment. Significant correlations between administrations (r = 0.65-0.70) were found suggesting substantial across-time consistency in responses (Tanosky-Kraff et al., 2008b). 13 The Emotional Eating Scale- Adapted for Children and Adolescents (EES-C; TanofskyKraff et al., 2007) is a 26-item, self-report questionnaire designed to assess the urge to cope with negative affect through eating. This questionnaire was adapted from the adult Emotional Eating Scale (EES) for use with children and adolescents ages 8 to 17. Participants are presented with a list of emotions (e.g., resentful, discouraged, worn out) and asked to rate on a 5-point scale their desire to eat from “I have no desire to eat” to “I have a very strong desire to eat.” A factor analysis generated three subscales that included eating in response to anxiety, anger, and frustration (e.g., furious, worried), eating in response to depressive symptoms (e.g., down, sad), and eating in response to feeling unsettled (e.g., excited, resentful). Excellent internal consistency was observed for each subscale with Cronbach alphas of 0.83-.93 (Anxiety, Anger and Frustration subscale), 0.92 (Depression subscale), and 0.83 (Unsettled subscale) (TanoskyKraff et al, 2007). The EES-C questionnaire was found to have good temporal stability over an average of three months with intraclass correlations ranging from 0.59-0.74 depending on subscale (Tanofsky-Kraff et al., 2007). Pubertal Development In addition to the primary analyses described below, I also explored whether socialization and selection effects vary by pubertal development in the pre-adolescent and adolescent twins. Previous data has shown increased phenotypic (Klump et al., 2013) and genetic risk (Culbert, Burt, McGue, Iacono, & Klump, 2009; Klump et al., 2000; Klump, Burt, McGue, & Iacono, 2007) for disordered eating symptoms during puberty. In the current study, these pubertal changes could translate into differences in associations between body conscious peer groups and disordered eating across pubertal development. In particular, socialization effects may be more prominent before puberty when there is smaller genetic influences on disordered eating (Klump, 14 McGue, & Iacono, 2003), while selection effects may be more prominent after puberty when genetic influences on disordered eating is increased. I used the Pubertal Development Scale (PDS; Peterson, Crockett, Richards, & Boxer, 1988) to examine pubertal development. The PDS is a self-report questionnaire, which asks the participant to assess their pubertal development based on physical markers of puberty (i.e., height spurts, body hair growth, skin changes, breast development, onset of menarche). Participants rated the development of these physical markers on a 4-point scale: (1) development has not yet begun; (2) development has barely started; (3) development is definitely underway; and (4) development seems completed. An exception to this 4-point scale is the coding for menses, which is coded dichotomously. The ratings of each physical marker will be summed and averaged to obtain an overall PDS score, with higher score representing more advanced pubertal status. The PDS exhibits good psychometric properties and the PDS total score correlates highly (r = .61-.67) with physician ratings of pubertal development (Peterson et al., 1988). BMI Prior research has found BMI to be significantly associated with disordered eating (Jones, Bennett, Olmsted, Lawson, & Rodin, 2001; Keel, Fulkerson, & Leon, 1997). Additionally, peer group BMI has been significantly associated with individual BMI (Trogdon, Nonnemaker, &Pais, 2008), thus BMI was examined as a covariate in the association between body-conscious peer groups and disordered eating (see more info below). Height and weight were measured twice during the in-person assessment and the average height and weight were used to calculate BMI based on the following equation: weight (lbs) / [height (in)2] x 703. Data Analysis Data Preparation 15 Analyses were conducted separately for both twin and parent reports to explore whether associations between informants differ. I began my analysis by exploring intercorrelations between maternal and paternal reports on the disordered eating measures (i.e., MEBS, LOC-ED, EAH-C, EES-C). Significant intercorrelations, at least moderate in magnitude, would indicate parental reports should be averaged (i.e., maternal score + paternal score/2) to create a single parental report using methods established for other phenotypes (Burt, Larsson, Lichtenstein, & Klump, 2012). Due to the similarities in content of the body-conscious friend group questionnaires (see above), I also examined intercorrelations amongst the scales to determine potential overlap in constructs. High intercorrleations (i.e., approximately .75 or higher) would indicate the creation of a composite score that would provide an overall index of body conscious friend groups might be appropriate. Finally, disordered eating and body-conscious peer group data were log transformed prior to analyses to control for positive skew. Initial Associations After informant reports and scale intercorrelations were examined, I used Pearson correlations to investigate phenotypic associations between body conscious friend groups and disordered eating, specifically whether individuals with friend groups high in body/weight consciousness are significantly higher in disordered eating. Co-Twin Control Analyses Finally, this study aimed to examine whether associations between body conscious friend groups and disordered eating symptoms are due to purely socialization factors (i.e., higher body conscious friend groups lead to higher disordered eating) or whether selection effects play a role 16 (i.e., individuals who are more inclined towards disordered eating are more likely to choose body conscious friend groups). Co-twin control analyses were used to explore these possibilities. As noted previously, the co-twin control method uses within-twin pair discordance for body conscious friend groups to predict disordered eating symptoms in the twins. These analyses use regression-based models within a mixed linear model (MLM) approach. The use of twin data results in non-independence, given that twins would be expected to be more similar to one another than two unrelated same-aged females. MLM accounts for the non-independence of the twin data by nesting each individual twin within the twin pair. Separate MLMs were run to examine individual-level, within person effects (i.e., associations between body conscious friend groups and disordered eating within each twin) as well as within-twin pair effects (i.e., associations between co-twin discordance in body conscious peer groups and disordered eating in each twin) using the following equation: Yij = β0 + βW (xij - xi) + βBxi +εij where Yij is the observed outcome (level of disordered eating) for the jth twin (j= 1 or 2) in the ith twin pair (i= 1,2,…, N), β0 is the intercept term, βW is the within-pair effect of exposure (body-conscious peer groups), xij is the level of exposure for the jth twin in the ith twin pair, xi is the mean exposure index for the ith twin pair, βB is the between-pair effect of exposure (bodyconscious peer groups) , εij is the residual (correlated across 2 members of a twin pair). I Importantly, the within-twin pair effects are estimated in separate models for MZ and DZ twins, which allows for the identification of any potential selection effects and the source (i.e., genetic or shared environmental) of these effects. Specifically, if body conscious friend groups are causally linked to an increased rate of disordered eating by pure socialization effects, I would expect significant associations between differences in level of peer group and level of disordered 17 eating for all three groups in the model: MZ, DZ, and on the with-person level (see Scenario A in Figure 1). This would indicate the absence of selection effects (genetic or environmental) on the association between body conscious peers and disordered eating. Alternatively, if associations differ between the individual and MZ/DZ twin level, then there is evidence for selection effects. Significant associations at the individual level and in DZ twins, but not in MZ twins, would suggest that genetic factors play a role in the selection process (see Scenario B in Figure 1). Alternatively, significant associations between body conscious peer groups and disordered eating at the within-person level only (i.e., no significant associations in MZ or DZ twins) would indicate that both environmental and genetic influences play a role in the selection of body conscious peer groups by individuals with disordered eating symptoms (see Scenario C in Figure 1). All of these predictions are estimated within the MLMs using 1 predictor variable (i.e., exposure to weight-conscious peers) for the within-person model and 5 predictor variables for the MZ and DZ twin models. The five predictors in the twin models include the within-person effect (i.e., exposure to weight-conscious peers) as well as a within-twin pair effect (i.e., a difference score between each twin’s exposure value and the mean exposure score within the twin pair). A dummy coded variable (i.e., zygosity) that identifies MZ versus DZ twins is also included and is allowed to interact with both the exposure variable and the within-twin pair difference score in order to: 1) estimate between-family effects (i.e., zygosity x exposure to weight-conscious peers); and 2) test for significant differences in within-twin pair exposure effects between MZ and DZ twins (zygosity x within-twin pair difference scores). In the MZ twin model, the dummy coded zygosity variable lists the DZ twins as the control, whereas in the DZ twin model, the zygosity variable lists the MZ twins as the control. By running the models 18 twice with different dummy coded variables, the MLMs are able to obtain estimates of the within-pair exposure effect in both MZ and DZ twins (the primary variable of interest) as well as test for significant differences in the magnitude of these effects between MZ and DZ twins. Separate models were run with different disordered eating constructs as the outcome variable (i.e., body dissatisfaction, binge eating, weight preoccupation, MEBS total score, loss of control over eating, eating in the absence of hunger, emotional eating). As previously stated, if intercorrelations between body conscious friend group measures were high (i.e., approximately .75 or higher), measures would be combined using z scores. However, if measures were not significantly intercorrelated, each friend group measure would be examined in separate models. Exploratory analyses examined puberty as a moderator of associations – these models directly examined whether socialization or selection effects vary across pubertal development. In these models, there are 2 predictor variables for the within-person model and 11 predictor variables for the MZ and DZ models. The two predictors for the within-person model include the individual’s exposure to the weight conscious peer group and the individual’s pubertal status. Five of the predictor variables for the MZ and DZ models are described above (i.e., the within-person exposure to body conscious peer groups, co-twin differences in exposure, a dummy coded zygosity variable that indicates MZ versus DZ twins, the interaction between the zygosity variable and individual exposure, and the interaction between the zygosity variable and differences in twin exposure). In addition, these models included pubertal status as a predictor, as well as its interactions with the individual’s exposure to body conscious peer groups, the co-twin differences in exposure, the zygosity variable, and the 3-way interaction between pubertal status, zygosity and exposure. The interaction between pubertal status and co-twin differences in exposure was the critical test of whether the association between differential exposure and 19 disordered eating varies by pubertal status, while the interaction between pubertal status, the zygosity variable and the co-twin difference in exposure examined whether there are significant differences in the magnitude of the interaction between pubertal status and differences in exposure for MZ and DZ twins. Finally, all models (primary and exploratory) were conducted with and without age and BMI as covariates in order to ensure that results are not unduly influenced by these variables that are significantly associated with disordered eating (Jones et al., 2001; Keel, Fulkerson & Leon, 1997). Prior studies also have demonstrated an association between maternal disordered eating and daughter’s level of disordered eating (Pike & Rosdin, 1991; Ruther & Richman, 1993; Smolak, Levine & Schermer, 1999). The current study conducted analyses controlling for maternal disordered eating to explore whether results were affected. Due to the relatively large number of potential models to be examined, a conservative p value of .01 will be used for all analyses. 20 Results Scale Intercorrelations and Informant Reports Intercorrelations between scores on the four peer exposure questionnaires were significant and positive; however, they all tended to be moderate in magnitude and shared only 9-30% of their variance (see Table 1). The relatively limited amount of shared variance suggests that the questionnaires tap different aspects of body-conscious peer group exposure. Thus, cotwin control analyses were conducted separately for each peer group questionnaire score. Additionally, with the exception of weight preoccupation and eating in the absence of hunger (where r’s = .24-.32), correlations between maternal and paternal reports of the twin’s level of disordered eating were either non-significant or small in magnitude (i.e., r < 0.09) (see Table 2). A similar pattern of results was obtained for correlations between the twin reports and each parental report (i.e., r’s mainly < .16). Given the generally low levels of associations between twin and parental reports, the MLMs were examined separately for twin report, maternal report, and paternal report.1 Initial Phenotypic Associations between Disordered Eating and Exposure to Body Conscious Peer Groups Within-person, phenotypic associations between body conscious peer groups and disordered eating in the full sample are presented in Table 3. Significant, positive associations were found between all disordered eating scores and peer group scores; however, correlations varied in magnitude depending upon the questionnaires. Overall, stronger correlations were found between body conscious peer groups and the disordered eating symptoms assessed with 1 Studies of other phenotypes have used average scores (i.e., maternal report + paternal report/2), particularly when correlations approached .30. To ensure more robust correlations were not obtained using a composite score, correlations were conducted using combined parental report scores. Correlations remained mainly non-significant and low in magnitude. 21 the MEBS (e.g., total score, body dissatisfaction, weight preoccupation, binge eating; r’s = .25.48) than were observed with eating in the absence of hunger or emotional eating scales (i.e., r’s = .10-.32). Co-Twin Control Analyses Using Twin Report of Disordered Eating Symptoms Overall, findings from the co-twin control analyses using twin report suggested that within-person associations observed in Table 3 were accounted for by etiologic effects that are consistent with Scenario C in Figure 1. Despite significant and positive within-person associations between an individual’s level of exposure to body-conscious peer groups and disordered eating, associations between the within-pair difference in exposure and the individual’s level of disordered eating for MZ and DZ twins were generally small and nonsignificant (see Table 4). For the associations where significant within-pair effects were found, no significant differences between MZ and DZ within-pair parameters were observed, suggesting that the degree of association between peer exposure and disordered eating was similar in MZ and DZ twins. Because MZ twins are 100% matched on genetic material, the lack of significant association in this group suggests that genetic selection factors play a role in the association between body conscious peer groups and disordered eating. However, the presence of a similar pattern of effects in DZ twins (i.e., small and non-significant associations) suggests that something in addition to genetic factors must be accounting for this association. Since both MZ and DZ twins shared 100% of their shared environmental factors, lack of associations in both MZ and DZ twins suggest that shared environmental factors also likely play a role in the association between body-conscious peer groups and disordered eating. Notably, results 22 remained unchanged when controlling for age, BMI, pubertal status, and maternal disordered eating (see Tables 5 and 6)2. Interestingly, the pattern of results suggesting Scenario C described above was not found for associations between weight preoccupation and the two peer exposure questionnaires that focused on whether body weight and shape are important within the peer group (i.e., the Friends as a Source of Influence scale and Peer Attribution Scale). Within-person, results replicated those discussed previously by showing that exposure to body-conscious peer groups significantly predicted weight preoccupation. However, unlike those discussed previously, the within-pair difference in exposure also predicted each twin’s level of disordered eating for MZ and DZ twins (see Table 4). These significant effects for within-person and within-pair are consistent with Scenario A in Figure 1. These results provide support for socialization since even when controlling for genetic and shared environmental effects within-pairs, differential peer group exposure is significantly able to predict level of weight preoccupation in each twin. Co-Twin Control Analyses Using Parental Report of Twin’s Disordered Eating Symptoms The co-twin control analyses were also conducted using maternal and paternal report of disordered eating outcome measures. Results from maternal report on her daughter’s disordered eating are presented in Table 7, while paternal report results are presented in Table 8. Unlike findings for twin reports, there was a lack of within-person associations between disordered eating and exposure to body-conscious peer groups. Although correlations were in the expected direction, the magnitudes of the associations were generally small or non-significant. Further, 2 Due to a strong correlation between pubertal status and age (r=.76), the co-twin control models also were conducted separately for age and pubertal status. Results remained unchanged from those reported herein. 23 within-pair discordance was unable to significantly predict disordered eating outcomes (see Table 7 and Table 8). Thus, there was an overall lack of significant association between twins’ reported level of exposure to body conscious peer groups and parental report of disordered eating symptoms 24 Discussion Findings from this study are the first to suggest that associations between body-conscious peer groups and disordered eating may be due to selection factors rather than socialization effects. Results showed that girls who are higher on disordered eating (either due to genetic or shared environmental predispositions) appear to select into body-conscious peer groups rather than these peer groups causing increased disordered eating. Support for these types of genetic and shared environmental selection effects persisted even when controlling for age, BMI and pubertal status. This pattern was consistently observed across multiple disordered eating constructs (i.e., body dissatisfaction, weight preoccupation, binge eating, eating in the absence of hunger, emotional eating) and multiple measures of peer group exposure. Overall, these findings question the current predominate theory that the association between body-conscious peer groups and disordered eating is due entirely to socialization processes. Previously, researchers have emphasized socialization as the mechanism at work in the association between body-conscious peer groups and disordered eating. Studies examining the convergence or divergence of disordered eating behaviors with exposure to peer groups have provided support for socialization, in that individuals who spend more time together become more similar in their eating habits (Crandall, 1988; Zalta & Keel, 2006). However, these studies were unable to control for selection effects, since neither study accounted for whether the individuals were friends prior to living together in the sorority (Crandall, 1988) or living together as roommates (Zalta & Keel, 2006). Importantly, when peer groups were completely unselected, Myer & Waller (2001) observed a lack of convergence in disordered eating symptomology. Results from the present study corroborate these latter findings by supporting the presence of shared environmental and genetic selection effects in that when genetic and shared environment 25 influences were controlled for using the co-twin control method, there was a lack of association between exposure to body conscious peer groups and disordered eating. Aggregating findings across all studies, it is possible that genetic and shared environmental selection effects may drive who one chooses to affiliate with initially (e.g., who decides to join a sorority), and then socialization may work to increase the similarity within a particular peer group. This hypothesis would reconcile previous inconsistent findings and provide a more comprehensive model to understand the association between body-conscious peer groups and increased disordered eating symptomology. Future studies could use a longitudinal design to confirm these impressions and assess whether individuals at higher risk for disordered eating symptoms select into bodyconscious peer groups (i.e., demonstrating selection effects). This design then could assess whether after selecting into these peer groups, exposure leads to enhanced disordered eating symptoms across time (i.e., demonstrating socialization effects). If selection effects drive the initial entry into peer groups, it is important to identify the specific selection factors that may be at play. The current study controlled for age, pubertal development and BMI; however, the same pattern of results persisted, suggesting that the presence of selection effects does not vary by developmental stage or differences in body weight. Additionally, the pattern of results also remained unchanged when controlling for maternal disordered eating, suggesting that maternal disordered eating does not influence daughter’s selection into body-conscious peer groups. Due to prior research demonstrating an association between maternal disordered eating and daughter’s level of disordered eating, this finding was somewhat surprising. While it is possible that results may reflect that maternal disordered eating does not influence daughters’ selection into body-conscious peer groups, the current study examined a limited number of maternal disordered eating symptoms. Further, the symptoms that 26 were examined reflected more cognitive (i.e., body dissatisfaction, weight preoccupation) rather than behavioral aspects of disordered eating. It is possible selection effects may be more influenced by directly observable behaviors, such as maternal dieting, as young girls may be more likely to pick up and model overt rather than more covert (e.g., weight concerns) behaviors. Prior studies have provided support for maternal modeling of disordered eating symptoms (Pike & Rosdin, 1991; Ruther & Richman, 1993; Smolak, Levine & Schermer, 1999). This modeling within the home environment may lead young girls to select into peer groups who also exhibit these behaviors. Future studies are needed to more comprehensively assess objective maternal disordered eating behaviors to either replicate the non-association found in the present study or identify key aspects of maternal behavior that may contribute to selection effects. Personality factors may also drive selection into body-conscious peer groups, as these factors develop early in life prior to peer group selection. Prior twin studies have found that personality traits are heritable; with heritability estimates ranging from .39 to .58 (Tellegen et al., 1988, Jang, Livesley, Vernon, & Jackson, 1996). Additionally, twin studies have also demonstrated significant genetic and shared environmental effects for perfectionism (Tozzi et al., 2004). Perfectionism has been associated with an array of disordered eating symptoms (e.g., fasting, binge behaviors, etc.) (Forbush, Heatherton & Keel, 2007). Genetic and shared environmental influences on perfectionism may drive selection into body-conscious peer groups, such that young girls who are raised in an environment with excessively high standards or who are genetically predisposed to perfectionistic qualities may select into peer groups with other girls who are self-critical and have high standards. These self-critical girls may be more focused on achieving society’s overly emphasized notion of the “ideal” body. Thus, perfectionism may drive selection into self-critical, high achieving peer groups for girls who are inherently more 27 prone (e.g., via genetic and/or environmental predisposition to perfectionism) to preoccupation with achieving the “perfect” body. Zalta and Keel (2006) found that similar to bulimic symptoms, perfectionism was more similar between roommates who cohabitated during the academic year. However, unlike bulimic symptoms these similarities persisted over their summer break away. While it is unclear each individual’s initial level of perfectionism prior to cohabitation (i.e., roommates may have became more similar in their perfectionism with increased exposure), it is possible roommate selection was driven by similar levels of perfectionism. Future studies should use a longitudinal design to investigate the possibility of perfectionism driving selection into body-conscious peer groups by assessing an individual’s initial levels of perfectionism and disordered eating prior to joining the peer group and then after peer group exposure. While results from this study largely provided support for genetic and environmental selection effects across a variety of disordered eating symptoms (e.g., binge eating, body dissatisfaction, etc.), socialization effects were found for weight preoccupation for two of the peer group measures. It perhaps is not surprising that weight preoccupation, of all the constructs examined within this study, demonstrated socialization effects. The construct of weight preoccupation focuses on how much one thinks about body weight and shape. Thus, membership in a peer group that is preoccupied with body weight and shape would likely lead to more preoccupation within each individual, regardless of their initial level of weight preoccupation prior to joining the peer group. Other disordered eating symptoms (e.g., binge eating) may be less directly linked to peer group discussion of body weight and shape, as it is unclear why or how discussions of body weight and shape would directly lead to binge eating (versus leading to concerns about weight first, and then other behaviors (e.g., dieting) that might then lead to binge 28 eating). Importantly, however, only two of the four peer exposure scales demonstrated socialization effects. The two peer exposure questionnaires that demonstrated socialization effects tended to focus more on whether weight and shape are important within the peer group (i.e., the Friends as a Source of Influence scale and Peer Attribution Scale), while the two peer exposure questionnaires that demonstrated selection effects placed more of an emphasis on how often topics of weight and shape are discussed (i.e., the Perceived Friend Preoccupation with Weight and Dieting scale and the Appearance Conversions with Friends scale). It is unclear why these differences in peer group questionnaires would affect whether selection effects are present for weight preoccupation, as it seems likely that both the importance of body weight and shape and how often these topics are discussed would demonstrate similar associations to weight preoccupation. Additional research is needed to replicate the presence of socialization effects for weight preoccupation and to investigate differences in the type of peer group exposure that may result in socialization versus selection effects. Despite the many strengths of this study (i.e., the ability to test for selection effects, the use of multiple disordered eating constructs, etc.), this study was not without limitations. First, and perhaps most importantly, the co-twin control study design is unable to control for all environmental selection effects. The co-twin control model attempts to match individuals in the exposed and non-exposed groups on key characteristics. Thus, this design is able to control for genetic factors and shared environmental influences, as these factors are shared by co-twins. However, this design is unable to control for environmental factors that are different between twins, such as non-shared environment factors that are specific to each co-twin (e.g., twins participating in different sports or after school activities). Since these unique experiences vary across the twin pair, it is impossible to match twins on these experiences and thus, they cannot be 29 controlled for within the models. It is possible that these non-shared environmental factors are causing selection into peer groups with differing levels of focus on body weight and body shape (i.e., one co-twin plays soccer (a less weight-focused sport) while another takes ballet classes (a more weight-focused sport)). In addition to being unable to control for non-shared environmental effects, the co-twin control analyses are also unable to examine what portion of selection effects is due to genetic versus shared environmental selection effects. Since all co-twins within this sample were raised in the same rearing environment, they are matched 100% on their shared environment, i.e., MZ and DZ twins do not differ in their similarity for shared environmental effects. Therefore, there is no control group that is matched on genetic effects, but not matched on shared environment (MZ twins raised in separate homes would be such a control group) that could help identify shared environmental selection effects only. Despite these limitations, findings from the current study significantly advance the present literature, as no study to date as been able to disentangle selection from pure socialization effects and determine whether these selection effects are due to genetic predispositions only or genetic and shared environmental influences. Second, we were only able to examine twin reports of disordered eating, as associations were non-significant when using parental report of daughter’s disordered eating symptoms. This lack of association may be due to parents not being attuned to their daughter’s disordered eating symptoms. Past studies examining eating disorders and other internalizing symptoms suggest poor correspondence between child self report and parental report (Kolko & Kazdin, 1993); thus, it is likely that the lack of significant correlations between parental report and twin self-report are due to the difficulties parents have in “observing” internal symptoms, such as concern over body weight and shape. Additional research is needed to further explore the reliability and validity of 30 parental reported disordered eating symptoms and to replicate this study’s insignificant results. Other measures may better capture parental report of disordered eating, particularly measures that focus more strongly on behavioral and more observable aspects of disordered eating (e.g., loss of control over eating, compensatory behaviors). Third, data from only one informant (e.g., twin’s self report) was available for analyses of weight-focused peer groups. Thus, associations between body conscious peer groups and disordered eating found with twin self-reports may be inflated by shared method variance, such that similarities in response styles may have led to stronger associations between exposure and outcome variables than if different informants had reported on these variables (Podsakoff et al., 2003). Additional research is needed to explore whether similar associations and selection effects would be found when using twin report of disordered eating and parental or even peer reports of peer group exposure. Finally, since this study was conducted using a non-clinical sample, it is unknown if the findings from this study generalize to a clinical population. However, the disordered eating symptoms that were examined in this study are precursors to full clinical eating disorders (Killen, et al., 1996; Stice & Shaw, 2002), which suggest it is likely that similar results might be found in a clinical sample. One challenge in conducting the present study using a clinical sample would be finding a sufficient number of twins with clinical eating disorders to conduct a wellpowered twin analysis. In conclusion, while the predominate theory used to explain the association between body-conscious peer groups and disordered eating has focused on socialization, the current study is the first to support the presence of genetic and shared environmental selection effects. 31 Findings from this study suggest individuals at higher risk for developing disordered eating may select into high-risk environments, such as peer groups that place more of an emphasize on body weight and shape. Further research is needed to investigate specific factors that may drive selection into body-conscious peer groups. 32 APPENDICES 33 APPENDIX A Tables 34 Table 1 Intercorrelations between Scores on the Body Conscious Peer Group Scales (N = 383-388) Body Conscious Peer Group Measures Perceived Friend Preoccupation with Weight and Dieting Scale (PFP) PFP 1.00 ACF FSI PAS -- -- -- Appearance Conversations with Friends (ACF) 0.55** 1.00 -- -- Friends as a Source of Influence Scale (FSI) 0.53** 0.47** 1.00 -- Peer Attribution Scale (PAS) 0.39** 0.30** 0.33** 1.00 Note. Body conscious peer group scores were log-transformed prior to analyses. PFP= Perceived Friend Preoccupation with Weight and Dieting Scale; ACF= Appearance Conversations with Friends; FSI= Friends as a Source of Influence Scale; PAS=Peer Attribution Scale. **p < .001 35 Table 2 Pearson Correlations examining Associations between Twin, Maternal and Paternal Reports of Disordered Eating (Twin Report N=406-430; Maternal Report N=401-420; Paternal Report N= 140-142) Maternal & Paternal & Maternal & Twin Reports Twin Reports Disordered Eating Questionnaires Paternal Reports Minnesota Eating Behavior Survey (MEBS) Total Score 0.19** 0.05 0.07 0.09 0.13 0.03 Binge Eating 0.20** 0.16 0.02 Weight Preoccupation 0.10* -0.23* 0.24* Negative Affect 0.04 0.13 0.28** External 0.03 0.10 0.32** Fatigue/Boredom 0.08 <.01 0.24** Anxiety, Anger & Frustration 0.09 -0.10 -0.02 Depression 0.13* -0.06 -0.09 Unsettled 0.02 -0.06 0.01 Body Dissatisfaction Eating In the Absence of Hunger (EAH) Emotional Eating Survey (EES) *p< .05, ** p< .001 36 Table 3 Pearson Correlations examining Within-Person Associations between Body Conscious Peer Groups and Disordered Eating in the Full Sample using Twin Self-Report Body Conscious Peer Group Measures Disordered Eating Measures Perceived Friend Preoccupation with Weight and Dieting (N=365-387) Appearance Conversations with Friends (N=391-413) Friends as a Source of Influence (N=389-410) Peer Attribution Scale (N=389-411) Minnesota Eating Behavior Survey (MEBS) Total Score 0.45** 0.35** 0.45** 0.48** Weight Preocc. 0.41** 0.31** 0.43** 0.39** Body Dissatisfaction 0.38** 0.31** 0.25** 0.43** Binge Eating 0.35** 0.30** 0.36** 0.36** Eating in the Absence of Hunger (EAH) Negative Affect 0.24** 0.11* 0.27** 0.18** External Eating 0.23** 0.15** 0.20** 0.23** Fatigue/Boredom 0.32** 0.18** 0.22** 0.31** Emotional Eating Survey (EES) Anxiety, Anger and Frustration 0.17** 0.15** 0.21** 0.17** Depression 0.28** 0.24** 0.23** 0.26** Unsettled 0.16** 0.10* 0.15** 0.14** *p< 0.05, ** p<0.001 37 Table 4 Results from Co-Twin Control Analyses Examining Associations between Body-Conscious Peer Groups and Disordered Eating using Twin Self-Report Exposure Questionnaires Outcome Measures Perceived Friend Preoccupation with Weight and Dieting (N=369-387) Ex x Ind. MZ DZ Zyg (B) (B) (B) (B) Minnesota Eating Behaviors Survey Total Score 1.07* 1.11* 0.19 Body Diss. Binge Eating Weight Preocc Friends as a Source of Influence (N=392-410) Ex x Ind. MZ DZ Zyg (B) (B) (B) (B) Peer Attribution Scale (N=393-411) Ex x Ind. MZ DZ Zyg (B) (B) (B) (B) -0.91 0.71* 0.83* 0.25 -0.58 1.01* 0.72* 0.52 -0.20 1.02* 0.81* 0.83* 0.03 0.55* 0.48* 0.10 -0.38 0.39* 0.35 0.12 -0.23 0.39* 0.17 0.16 -0.01 0.61* 0.22 0.49* 0.27 0.54* 0.53* 0.15 -0.38 0.39* 0.46* 0.24 -0.22 0.56* 0.33 0.43 0.10 0.49* 0.27 0.54* 0.28 0.84* 0.81* 0.12 -0.69 0.49* 0.57* 0.07 -0.50 0.83* 0.55* 0.60* 0.05 0.73* 0.77* 0.56* -0.20 0.04 -0.09 0.04 0.05 0.02 -0.03 0.12* 0.06 0.08 0.02 0.08* 0.06 0.09 0.04 Eating in the Absence of Hunger Neg Affect 0.13 0.11* External Subsc Appearance Conversations with Friends (N=395-413) Ex x Ind. MZ DZ Zyg (B) (B) (B) (B) 0.19* 0.01 0.05 0.04 0.11* 0.12 -0.05 -0.17 0.16* 0.02 0.11 0.09 0.17* 0.10 0.22* 0.12 0.20* 0.16 -0.10 -0.26 0.09* -0.03 -0.08 -0.04 0.14* 0.07 0.01 -0.08 0.19* 0.15 0.17 0.02 Emotional Eating Scale Anx., Anger & Frustration 0.13* 0.08 0.09 0.02 0.09* 0.09 0.03 -0.06 0.14* 0.19 0.11 -0.08 0.11* 0.05 0.21* 0.16 0.22* 0.08 0.18 0.09 0.15* 0.05 0.10 0.04 0.17* 0.19 0.19 0.00 0.17* 0.04 0.27* 0.23 0.13* 0.03 0.07 0.04 0.07 -0.09 0.11 0.21 0.12* 0.12 0.12 -0.01 0.10* 0.02 0.12 0.11 Fatigue Depression Unsettled 38 Table 4 (cont’d) Note. B = fixed effects beta estimates from the mixed linear models that index how effectively exposure to body-conscious peer groups can predict the outcome (i.e., disordered eating) in each set of analysis. “Individual” = within-person associations between body conscious peer groups and disordered eating. “MZ” = within-pair association, or the ability for differences in peer exposure to body conscious peer groups between co-twins to predict each twin’s level of disordered eating for only MZ twins pairs. Similarly, “DZ” = within-pair association for only DZ twins. “Ex x Zyg” indicates whether there are significant differences in within-pair effects for MZ and DZ twins *p< .01 39 Table 5 Results from Co-Twin Control Analyses Examining Associations between Body-Conscious Peer Groups and Disordered Eating using Twin Self-Report Controlling for Age, BMI and Pubertal Status Exposure Questionnaires Perceived Friend Preoccupation with Weight and Dieting (N=294) Ex x Ind. MZ DZ Zyg Outcome (B) (B) (B) (B) Measures Minnesota Eating Behaviors Survey Appearance Conversations with Friends (N=294) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Friends as a Source of Influence (N=313) Ind. (B) MZ (B) DZ (B) Peer Attribution Scale (N=313) Ex x Zyg (B) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Total Score 1.03* -0.34 0.57 0.91 0.71* -0.60 0.34 0.94 0.99* -0.76* 0.21 0.97 0.98* -0.06 0.22 0.28 Body Diss. 0.42* -0.07 -0.32 -0.25 0.32* -0.09 -0.15 -0.06 0.33* -0.28 -0.02 0.27 0.49* -0.03 -0.24 -0.21 Binge Eating 0.51* -0.28 0.47 0.75 0.36* -0.30 0.40 0.70 0.57* -0.47* -0.02 0.45 0.53* -0.19 -0.10 0.09 Weight Preocc. 0.82* -0.59 0.40 0.99 0.55* -0.49 0.28 -0.78 0.84* -0.42 0.44 0.86 0.70* 0.27 0.32 -0.05 Eating in the Absence of Hunger Neg Affect 0.11* -0.12 -0.01 0.12 0.03 -0.06 0.05 0.11 0.11* 0.04 -0.13* -0.17 0.10* 0.06 -0.05 -0.12 External Subsc 0.15* -0.33 0.07 0.41 0.05 -0.11 0.06 0.17 0.14* -0.11 -0.06 0.06 0.17* -0.15 -0.01 0.14 Fatigue 0.16* 0.03 0.06 0.03 0.03 -0.03 0.05 0.08 0.12* 0.06 -0.16 -0.22 0.19* 0.04 -0.11 -0.15 -0.07 0.02 0.09 0.10* -0.12 -0.02 0.10 0.14* -0.05 -0.16 -0.11 0.14* -0.23* -0.08 0.14 Emotional Eating Scale Anger, Anx, & 0.13* Frustration Depression 0.18* 0.02 <0.01 -0.02 0.12* -0.13 -0.02 0.11 0.16* -0.01 -0.14 -0.13 0.20* -0.20 -0.01 0.19 Unsettled 0.14* -0.06 0.20 0.26 0.05 -0.11 <0.01 0.11 0.12* 0.08 -0.06 -0.13 0.12* <0.00 -0.06 -0.06 40 Table 5 (cont’d) Note. B = fixed effects beta estimates from the mixed linear models that index how effectively exposure to body-conscious peer groups can predict the outcome (i.e., disordered eating) in each set of analysis. “Individual” = within-person associations between body conscious peer groups and disordered eating. “MZ” = within-pair association, or the ability for differences in peer exposure to body conscious peer groups between co-twins to predict each twin’s level of disordered eating for only MZ twins pairs. Similarly, “DZ” = within-pair association for only DZ twins. “Ex x Zyg” indicates whether there are significant differences in within-pair effects for MZ and DZ twins *p<.01 41 Table 6 Results from Co-Twin Control Analyses examining Associations between Body-Conscious Peer Groups and Disordered Eating using Twin Self-Report Controlling for Age, BMI and Maternal Disordered Eating Exposure Questionnaires Outcome Measures Perceived Friend Preoccupation with Weight and Dieting (N=330) Ex x Ind. MZ DZ Zyg (B) (B) (B) (B) Appearance Conversations with Friends (N=353) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Friends as a Source of Influence (N=351) Peer Attribution Scale (N=352) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Minnesota Eating Behaviors Survey Total Score 1.14* -0.37 0.65 1.02 0.69* -0.53 0.35 0.89 1.09* -0.81* 0.23 1.04* 1.10* -0.15 0.32 0.47 Body Diss 0.51* -0.04 -0.25 -0.20 0.29* 0.03 -0.08 -0.11 0.38* -0.29 -0.23 0.06 0.64* -0.09 -0.19 -0.10 Binge Eating 0.55* -0.32 0.43 0.74 0.36* -0.25 0.32 0.58 0.61* -0.47* 0.23 0.57 0.58* -0.24 -0.10 0.14 Weight Preocc 0.88* -0.61 0.50 1.11* 0.56* -0.49 0.29 0.78 0.89* -0.43 0.50 0.93* 0.74* 0.23 0.35 0.13 Eating in the Absence of Hunger Neg Affect 0.11* -0.12 <0.01 0.13 0.03 -0.06 0.03 0.08 0.11* 0.04 -0.06 -0.10 0.10* 0.06 -0.04 -0.10 External Subsc 0.15* -0.33 0.08 0.40 0.06 -0.12 0.05 0.17 0.15* -0.12 -0.06 0.06 0.17* -0.15 <0.01 0.16 Fatigue 0.17* 0.03 0.05 0.02 0.04 -0.06 0.03 0.10 0.15* 0.04 -0.15 -0.19 0.19* 0.06 -0.08 -0.13 -0.08 0.02 0.10 0.11* -0.14 -0.01 0.13 0.14* -0.05 -0.10 -0.05 0.14* -0.23* -0.07 0.16 Emotional Eating Scale Anger, Anx, & 0.13* Frustration Depression 0.18* 0.02 -0.01 -0.02 0.12* -0.14 <0.01 0.14 0.16* -0.01 -0.05 -0.04 0.20* -0.21 -0.01 0.19 Unsettled 0.13* -0.06 0.24 0.30 0.07 -0.14 0.01 0.15 0.12* 0.06 0.01 -0.05 0.11* 0.03 -0.05 -0.08 42 Table 6 (cont’d) Note. B = fixed effects beta estimates from the mixed linear models that index how effectively exposure to body-conscious peer groups can predict the outcome (i.e., disordered eating) in each set of analysis. “Individual” = within-person associations between body conscious peer groups and disordered eating. “MZ” = within-pair association, or the ability for differences in peer exposure to body conscious peer groups between co-twins to predict each twin’s level of disordered eating for only MZ twins pairs. Similarly, “DZ” = within-pair association for only DZ twins. “Ex x Zyg” indicates whether there are significant differences in within-pair effects for MZ and DZ twins *p< .01 43 Table 7 Results from Co-Twin Control Analyses examining Associations between Body-Conscious Peer Groups and Disordered Eating Using Maternal Report of Twin’s Disordered Eating Symptoms Exposure Questionnaires Outcome Measures Perceived Friend Preoccupation with Weight and Dieting (N=361-379) Ex x Ind. MZ DZ Zyg (B) (B) (B) (B) Appearance Conversations with Friends (N=384-403) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Friends as a Source of Influence (N=381-400) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Peer Attribution Scale (N=382401) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Minnesota Eating Behaviors Survey Total Score 0.59* -0.26 0.40 0.65 0.31* -0.09 0.19 0.28 0.41* 0.05 0.50* 0.46 0.47* 0.23 0.30 0.07 Body Diss. 0.56* -0.10 0.59* 0.70 0.32* 0.19 0.20 0.01 0.40* 0.03 0.66* 0.62 0.40* 0.10 0.34 0.23 Binge Eating 0.18* -0.13 -0.06 0.07 0.08 -0.05 <0.01 0.08 0.01 -0.06 0.07 0.23* 0.09 0.30* 0.21 Weight Preocc 0.36* -0.13 0.25 0.38 0.15 -0.10 0.04 0.15 0.17 0.12 0.04 -0.08 0.22* 0.25 -0.10 -0.35 -0.05 Eating in the Absence of Hunger Neg Affect 0.05 -0.02 -0.06 -0.04 0.04 0.01 -0.01 -0.02 0.04 0.03 -0.02 -0.05 0.07* 0.02 0.11* -0.10 External Subsc <0.01 -0.10 -0.05 0.05 <0.01 -0.03 -0.04 -0.01 0.02 <0.01 -0.05 -0.05 0.05 0.03 0.07 0.04 Fatigue <0.01 -0.01 -0.10 0.08 0.01 0.03 -0.09 -0.11 0.01 -0.03 -0.03 <0.01 0.06* <0.01 0.14* 0.14 0.04 -0.06 -0.09 <0.01 0.01 -0.03 -0.04 0.02 <0.01 0.02 0.02 0.02 0.01 0.03 0.02 Emotional Eating Scale Anger, Anx, & 0.02 Frustration Depression 0.04 <0.01 -0.06 -0.06 0.03 0.01 -0.02 -0.03 0.02 -0.02 -0.01 0.01 0.08* 0.01 0.12* 0.11 Unsettled 0.05 0.09 -0.09 -0.18 0.03 0.04 0.01 -0.03 0.02 -0.03 0.02 0.05 0.03 0.05 <0.01 -0.05 44 Table 7 (cont’d) Note. B = fixed effects beta estimates from the mixed linear models that index how effectively exposure to body-conscious peer groups can predict the outcome (i.e., disordered eating) in each set of analysis. “Individual” = within-person associations between body conscious peer groups and disordered eating. “MZ” = within-pair association, or the ability for differences in peer exposure to body conscious peer groups between co-twins to predict each twin’s level of disordered eating for only MZ twins pairs. Similarly, “DZ” = within-pair association for only DZ twins. “Ex x Zyg” indicates whether there are significant differences in within-pair effects for MZ and DZ twins *p< .01 45 Table 8 Results from Co-Twin Control Analyses examining Associations between Body-Conscious Peer Groups and Disordered Eating Using Paternal Report of Twin’s Disordered Eating Symptoms Exposure Questionnaires Perceived Friend Preoccupation with Weight and Dieting (N=125-126) Ex x Ind. MZ DZ Zyg Outcome (B) (B) (B) (B) Measures Minnesota Eating Behaviors Survey Appearance Conversations with Friends (N=133-135) Friends as a Source of Influence (N=133-135) Peer Attribution Scale (N=131-133) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Ind. (B) MZ (B) DZ (B) Ex x Zyg (B) Total Score 0.34 0.28 0.09 -0.18 0.31 0.52 0.12 -0.40 0.03 -0.11 -0.29 -0.19 0.32 0.62 -0.08 -0.70 Body Diss. 0.29 0.15 0.28 0.13 0.28* 0.58* 0.21 -0.37 0.12 -0.19 0.18 0.36 0.30* 0.60 0.08 -0.52 Binge Eating 0.14 0.32 0.07 -0.25 0.04 0.01 0.03 -0.08 -0.10 -0.05 0.05 0.04 -0.28 -0.10 0.18 Weight Preocc 0.22 -0.40 0.29 0.70 0.06 -0.12 -0.07 0.05 0.05 -0.09 0.05 0.14 0.19 0.61 -0.02 -0.62 0.02 Eating in the Absence of Hunger Neg Affect -0.01 -0.13 -0.05 0.08 0.02 <0.01 0.01 0.01 -0.06 -0.14* -0.04 0.10 0.02 0.07 0.01 -0.07 External Subsc 0.05 0.06 0.02 -0.04 0.04 0.05 0.06 <0.01 -0.04 <0.01 -0.17 -0.18 0.01 0.09 -0.05 -0.14 Fatigue -0.04 -0.07 -0.12 -0.04 0.04 0.03 0.02 <0.01 0.11* -0.25* -0.12 0.13 -0.01 <0.01 0.02 0.02 Emotional Eating Scale Anger, Anx, & Frustration -0.01 0.07 -0.08 -0.15 <0.01 <0.01 0.02 0.02 -0.02 0.01 -0.06 0.07 -0.02 -0.04 -0.01 0.03 Depression -0.01 -0.06 0.04 0.10 -0.03 -0.11 0.08 0.19* -0.03 -0.05 -0.01 0.04 -0.01 -0.05 0.05 0.10 Unsettled -0.01 -0.01 0.01 0.02 -0.05 -0.07 -0.03 0.03 -0.05 -0.08 -0.02 0.06 0.06 -0.13 -0.03 0.11 46 Table 8 (cont’d) Note. B = fixed effects beta estimates from the mixed linear models that index how effectively exposure to body-conscious peer groups can predict the outcome (i.e., disordered eating) in each set of analysis. “Individual” = within-person associations between body conscious peer groups and disordered eating. “MZ” = within-pair association, or the ability for differences in peer exposure to body conscious peer groups between co-twins to predict each twin’s level of disordered eating for only MZ twins pairs. Similarly, “DZ” = within-pair association for only DZ twins. “Ex x Zyg” indicates whether there are significant differences in within-pair effects for MZ and DZ twins *p< .01 47 APPENDIX B Figures 48 Association Between Body Conscious Peer Groups and Disordered Eating Individual level Within DZ pairs Within MZ pairs Scenario A Scenario B Scenario C Figure 1. Summary of Potential Results of Co Co-Twin Control Analysis. 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