Natural cohorts: family similarity in adult cognition.

We now return to the sets of hypotheses formulated in the introductory section to explore how the findings reported above bear on these propositions. Our first hypothesis dealt with whether family similarity could be observed in adulthood and whether there were differing ability patterns in such similarity. Significant family similarities were observed for our total sample for all ability measures, except Perceptual Speed, and for the cognitive style measures. The magnitude of correlations for the ability measures are comparable for those found between young adults and their children (DeFries et al., 1976). Similar to the DeFries study, we also found differences in resemblance across subsets. For example, same gender pairs showed higher correlations on Verbal Meaning, Number, and Word Fluency but opposite-gender pairs on Spatial Orientation, Inductive Reasoning, and Motor-Cognitive Flexibility. Also, greater similarity was found between mother-offspring pairs than father-offspring pairs on Inductive Reasoning and Psychomotor Speed. Moreover, higher parent-offspring correlations were found for daughters than for sons, suggesting at least the possibility that females may experience greater shared environmental influences than males. Our first hypothesis also argued for the possible effect of early shared environment upon offspring performance on Verbal Meaning and Word Fluency. After age adjustment, these were indeed the abilities that showed the highest parent-offspring similarity. Our second hypothesis proposed that if shared environmental influences are relatively unimportant in adulthood, then similarity within parent-offspring pairs should remain reasonably constant in adulthood across time and age. Our examination of this issue with a longitudinal sample ranging over a 21-year period strongly supports this proposition for all of those variables that displayed significant parent-offspring correlations. Indeed, parent-offspring correlations measured at approximately the same age of parent and offspring and when those ages were 20 years apart had similar magnitudes. The third hypothesis asked whether family similarity would decrease with age because of the increasing amount of nonnormative, nonshared environment expected as adult life progresses. Counterintuitively, no such decrease in similarity could be observed. Indeed, for two variables there was evidence for increasing similarity as a function of offspring age. This finding makes good sense for our Perceptual Speed variable. Most of our younger offspring typically have not yet experienced age-related decline on this variable, whereas some of their parents have. Both older offspring and parents may have experienced sufficient decline so that once again their observed similarity is increased.(ABSTRACT TRUNCATED AT 400 WORDS)