Abstract
Preservation of cortical brain tissue is often viewed as the foundation of successful cognitive aging. However, this association could instead reflect a lifelong association between cognitive ability and cortical tissue. We analyzed cortical data on 588 subjects from the Lothian Birth Cohort 1936 who have IQ scores from the same test available at 11 and 70 years as well as brain MRI at 73 years. Childhood IQ accounted for more than two-third of the association between IQ and cortical thickness in old age. This warns against ascribing a causal interpretation to the association between cognitive ability and cortical tissue in old age based on assumptions about, and exclusive reference to, the aging process. Without early-life measures of cognitive ability, it would have been tempting to conclude that preservation of cortical thickness in old age is a foundation for successful cognitive aging when, instead, it is a lifelong association.