Genetic and environmental influences on the development of intelligence.

Measures of intelligence were collected in 209 twin pairs at 5, 7, 10, and 12 years of age, as part of a longitudinal project on intelligence, brain function, and behavioral problems. Intelligence was measured at 5, 7, and 10 years of age with the RAKIT, a well-known Dutch intelligence test, consisting of 6 subscales. At 12 years of age, the complete WISC-R was administered (12 subscales). Both intelligence tests resulted in a measure of full-scale IQ (FSIQ). Participation rate is around 93% at age 12. Correlation coefficients over time are high: (r(5-7) = .65; r(5-10) = .65; r(5-12) = .64; r(7-10) = .72; r(7-12) = .69 and r(10-12) = .78). Genetic analyses show significant heritabilities at all ages, with the expected increase of genetic influences and decrease of shared environmental influences over the years. Genetic influences seem to be the main driving force behind continuity in general cognitive ability, represented by a common factor influencing FSIQ at all ages. Shared environmental influences are responsible for stability as well as change in the development of cognitive abilities, represented by a common factor influencing FSIQ at all ages and age-specific influences, respectively.