Genetic influences and individual differences in development of formal operations

Abstract

A study of formal operational ability in individuals from same sex twin pairs was conducted utilizing the twin methodology of behavior genetics to determine whether attainment of this Piagetian stage could contribute information about the relative influence of heredity and environment on cognitive development. Analysis of the formal operation stage as related to psychometric measurements of nonverbal general intelligence (inductive reasoning) and spatial visualization was undertaken in order to further define the process characterizing this cognitive level. Three formal operational tasks (Inhelder & Piaget, 1958) were individually administered to assess developmental level, the Advanced Progressive Matrices (Raven, Court & Raven, 1978) to assess intelligence, and the Mental Rotations Test (Vandenberg, 1975) to assess spatial visualization. Information about experiences contributing to individual (especially sex) differences in attainment of formal operations was sought through interviews. The 54 subjects (38 females and 16 males) of the study were from 17 pairs of identical (MZ) and 10 pairs of fraternal (DZ) twins, from 9 years of age to 54, from a southwestern United States urban area. Considerable individual variation on al measures was observed; intense within-family similarity and extensive between-family differences of comparable magnitude and order in both developmental and psychometric assessments indicated strong between-family environmental influences. Although intrapair variation in MZ twins was consistently less than in DZ twins, lack of statistically significant differences between the two twin types limited inferences about genetic influence. Piagetian stage development was related more strongly to intelligence than age and to spatial visualization and an accuracy performance style.