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OBJECTIVE: Age differences in episodic memory (memory) have been attributed to a general reduction in processing speed (the "speed mediation hypothesis"), but also to declines in the efficiency of executive functions operations ("executive decline hypothesis"). To test predictions from these competing models, we examined the mediating effects of processing speed (speed) and executive functions (executive) on age and episodic memory in three older adult cohorts. METHOD: The first sample comprised 842 individuals from the Sydney Memory and Aging Study (MAS). The second and third samples included 476 individuals from the Older Australian Twins Study (OATS), with each twin from a pair randomly assigned to form two samples. A series of regression analyses was performed on each of the three samples independently, so as to obtain the sizes and statistical significances of the indirect effects of age on each of the memory variables, mediated by each of the Executive and Speed composites. Sex was a control variable for all analyses. Analyses were repeated with current depression as an additional control variable. RESULTS: Data from the MAS sample suggested that both Speed and Executive composites were significant mediators, with the former having a stronger mediation effect. A similar pattern was found in the two OATS samples. CONCLUSIONS: These findings are consistent with those of previous studies in which speed had a stronger mediating effect than executive on age-related variation in memory. They provide further support for the speed mediation hypothesis, although not negating the executive decline hypothesis.
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