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One of the central concepts within the literature on cognitive aging is the notion of dedifferentiation—the idea that increasing age is associated with an increase in the interrelatedness of different cognitive abilities. Despite the centrality of this dedifferentiation hypothesis, there is a great deal of evidence that both supports and does not support dedifferentiation. We hypothesized that these inconsistent findings were due to (a) the use of different cognitive abilities (i.e., memory vs. speed of processing) that were correlated; and (b) the differing age groups that were used across studies. By using data from 11 well-validated cognitive test batteries (K = 2,355, range of the mean ages of correlations 18–85+), we found evidence for linear dedifferentiation when a test assessing speed of processing was included in the correlation with test of other cognitive abilities. We speculate that previous findings of nonlinear dedifferentiation are likely a result of undiagnosed or unrecognized pathology in a subsample of participants.