Understanding the underlying mechanism of the spacing effect in verbal learning: a case for encoding variability and study-phase retrieval

The spacing effect refers to the mnemonic benefit of spacing repeated study events across time compared to massing (i.e. cramming) repeated study events. Due to the robustness of this finding, substantial research has been devoted to uncovering the spacing effect's underlying mechanism. Specification of such a mechanism has been guided by consistent findings across methodologies and several experimental manipulations that serve as boundary conditions. Past reviews of the spacing effect literature have generally considered subsets of these factors but never an exhaustive set. Thus, the current review considers more comprehensively six consistent findings in the extant spacing effect literature pertaining to human memory to better discriminate among the previously proposed theories. Review of the literature provides substantial evidence indicating the need for an encoding variability mechanism [e.g. Glenberg, A.M.(1976). Monotonic and nonmonotonic lag effects in paired-associate and recognition memory paradigms. Journal of Verbal Learning and Verbal Behavior, 15, 1-16] in addition to a reminding mechanism [e.g. Benjamin, A.S., & Tullis, J.G.(2010). What makes distributed practice effective? Cognitive Psychology, 61, 228-247].