Secondary task engagement drives the McCabe effect in long-term memory.

Processing that occurs while information is held in working memory is critical in long-term retention of that information. One counterintuitive finding is that the concurrent processing required during complex span tasks typically impairs immediate memory, while also leading to improved delayed memory. One proposed mechanism for this effect is retrieval practice that occurs each time memory items are displaced to allow for concurrent processing during complex span tasks. Other research has instead suggested that increased free time during complex span procedures underlies this effect. In the present study, we presented participants with memory items in simple, complex, and slow span tasks and compared their performance on immediate and delayed memory tests. We found that how much a participant engaged with the secondary task of the complex span task corresponded with how strongly they exhibited a complex span boost on delayed memory performance. We also probed what participants were thinking about during the task, and found that participants' focus varied depending both on task type and secondary task engagement. The results support repeated retrieval as a key mechanism in the relationship between working memory processing and long-term retention. Further, the present study highlights the importance of variation in individual cognitive processing in predicting long-term outcomes even when objective conditions remain unchanged.