Cognition Studies of secular trends in cognition have focused almost exclusively on unrelated cohorts. The study of biologically related generations is important for several reasons. First, comparison of cohort versus generational data permit examination of whether a similar increase in prevalence of positive developmental trajectories hypothesized to occur across cohorts is also found across generations. More important, the comparison of the relative impact of neurobiological versus sociocultural influences, in biologically related individuals versus cohorts, would inform the relative potency of cultural and genetic influences on intelligence at various developmental periods. For example, the co-constructionist perspective posits that the influence of neurobiological factors increases in old age and exceeds the impact of cumulative cultural influences. A more stringent test of the increased impact of neurobiological factors in old age should be a study of successive family generations in contrast to successive unrelated cohorts given the shared genetics and environment across generations. The increased influence of neurobiological factors in old age is based in part on the assumption among evolutionary theorists that positive selection effects are most clearly manifested early in the lifespan and that the expression of deleterious genes in old age has been less constrained by the evolutionary process ( Finch & Kirkwood, 2000 ).