The spacing/lag effect refers to the finding in memory research that spacing repeated study more widely produces important learning benefits (Crowder, 1976; Dempster, 1988, 1989). In order to know when and how this effect can be most useful for second language learning, it is important to understand the cognitive mechanism(s) that drive any effects of spacing in second language learning. It is also important to understand how the operation of the mechanism(s) may be affected by variables inherent in second language learning contexts. In the present study, I investigate the contribution of the dual mechanism of effortful successful retrieval to the effects of lag in second language vocabulary learning. This dual mechanism is proposed to underlie both beneficial and detrimental effects of lag on learning within the reminding account (Benjamin & Tullis, 2010). I additionally investigate the potential effects of externally imposed study time on learning as well as on the operation of the two mechanisms under investigation.Fifty-two native speakers of American English studied 72 novel L2 Finnish words during overt oral L2-L1 translation retrieval practice in a paired-associate learning format from 6 repetitions under three constant levels of within-session lag with immediate study of feedback for 3 or 9 seconds after each retrieval attempt. Study-phase response latencies and accuracy were recorded and used as measures of study-phase retrieval effort and success, respectively (as in Maddox & Balota, 2015). Immediate and delayed form recognition, L2-L1 translation and translation matching posttests were used to measure learning outcomes.Results showed a large spacing effect on all measures and at both times of test administration as well as a lag effect on delayed meaning tests. A nonsignificant nonmonotonic function was observed only on immediate posttests. Study time had an overall small positive effect on learning; however, it did not cancel out negative effects of massing retrieval practice: the effects of spacing were considerably larger. Increasing lag between retrieval attempts produced increasingly longer study-phase response latencies and increasingly lower levels of study-phase retrieval success. Study time had a small nonsignificant negative effect on study-phase response latencies and a small significant positive effect on study-phase retrieval success. Moderated mediation analyses showed that study time, as operationalized in the present study, did not affect the operation of the two underlying mechanisms under investigation. They further showed that, despite the fact that a nonmonotonic function was not observed in the present learning outcomes, increasing inter-study interval still had a negative effect on learning and this effect operated through a lower rate of study-phase retrieval success. Further, the moderated mediation analyses showed that the positive effects of retrieval effort (Roediger & Karpicke, 2006) were conditional on retrieval success, in line with predictions of the reminding account.The findings of the dissertation suggest that: (a) massed L2-L1 translation retrieval practice may not be effective for L2 vocabulary learning; (b) externally imposing a longer study time does not have the large benefits that learner-regulated longer study time does; (c) effortful successful retrieval underlies benefits of lag in L2 vocabulary learning from L2-L1 retrieval practice-the benefits of effortful retrieval are conditional on retrieval success, even in the presence of immediate feedback; (d) successful retrieval is more beneficial than unsuccessful retrieval, even when retrieval attempts are followed by immediate feedback - study of feedback does not offset the negative effects of retrieval failure.
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