How strong is the evidence for mediational hypotheses of age-related memory loss?

Background: Luszcz and Bryan review research supporting three theories of age-related memory loss: the speed hypothesis, the executive function hypothesis, and the common cause hypothesis. Objective: The aim of this commentary is to extend that review by encouraging consideration of the strength (or lack thereof) of the empirical evidence supporting theories of age-related memory loss. Methods: Arguments are presented that call into the question the strength of the evidence that derives from cross-sectional analysis of individual difference sources of variance. Results: Supporting evidence for mediational hypotheses of cognitive aging (1) derives from potentially ambiguous statistical techniques; (2) is based on untested assumptions about the between and within person sources of variance; (3) is not supported by longitudinal studies, and (4) relies heavily on arguments of parsimony. Conclusions: Existing evidence is not strong enough to grant any particular theory presumptive status. We concur with Luszcz and Bryan that supplementing the now popular individual differences research designs with alternative approaches would advance theory development and testing.