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The author makes three principal assertions in his artircle:
First, that it is possible to draw conclusions about of age of onset of age decrement in cognition from cross-sectional studies.
Second, that positive age changes reported in the literature on longitudinal studies represent the short-term effects of practice.
Third, that the onset of cognitive decline occurs in the 20s for many abilities.
I would like to suggest that the author has not provided empirical support for ant of these assertions in the data reported in this article or other of his publications cited, and that these assertion contradict empirical findings and thoretical arguments presented in the literature on cognitive aging over the past half century, as follows.
Salthouse continues to reify the “cross-sectional fallacy (cf. Riley, 1973), that age changes within individuals over time (and age) can be inferred from cross-sectional age differences between groups of individuals of different agents measured at the same point time. Ovet 40 years ago the formal reasons why this is not possible were explicated in sociology by Ryder (1965) and in psychology by Schaie (1965). Both these authors explicated that age changes and age differences could be identical only if there was a perfectly stable environment over time and no differences in the level of performance between successive birth cohorts at the same age (also cf. Hofer, Sliwinski & Flaherty, 2002). Neither Salthouse nor anyone else has ever provided empirical data that would meet these conditions! (Schaie & Hofer, 2001).
Some additional concerns:
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