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Unformatted text preview: 0001-8244/02/0700-0237/0 © 2002 Plenum Publishing Corporation Behavior Genetics, Vol. 32, No. 4, July 2002 (© 2002) 237 Genetic and Environmental Influences on the Development of Intelligence M. Bartels, 1,2 M. J. H. Rietveld, 1 G. C. M. Van Baal, 1 and D. I. Boomsma 1 Received 11 Jan. 2002—Final 24 Apr. 2002 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 sub- scales). 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) 5 .65; r(5–10) 5 .65; r(5–12) 5 .64; r(7–10) 5 .72; r(7–12) 5 .69 and r(10–12) 5 .78). Genetic analyses show sig- nificant 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. KEY WORDS: General cognitive ability; longitudinal analyses; heritability; twin study; simplex. 1993; Boomsma, 1993; Plomin et al., 1997; Boomsma and Van Baal, 1998; Alarcón, 1998, 1999). A few lon- gitudinal studies have focused on the influences of genes and environment on cognitive development rather than cognition at specific ages. New genetic in- fluences at different ages and a common factor for shared environmental influences have been found (Col- orado Adoption Project; e.g., Plomin and DeFries, 1985; Louisville Twin Study; e.g., Wilson, 1983; Eaves et al., 1986). Longitudinal twin and family data allow the study of persistence and change of genetic, shared environ- mental, and unique environmental influences. The ge- netic and environmental influences may exert their effects following several possible mechanisms. First, ge- netic or environmental factors may exert a continuous influences from their time of onset (common factor in- fluences). This mechanism implies that the same genetic or environmental factors are responsible for stability, possibly with age-dependent factor loadings. Second, ge- netic and environmental influences may be specific at a certain age and exert an effect on cognition at that age INTRODUCTION Heritability of intelligence has been studied exten- sively, both in adults and in children, but far less is known about the developmental genetics of cognitive abilities. Many behavior genetic studies yield theabilities....
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