Marcie Chambers1 & D. W. Fulker1.
A Cholesky decomposition model was used to analyze IQ and academic achievement in twin pairs. IQ was measured by the Bayley Scales of Infant Development (New York: Psychological Corp) at ages 1 (N=717) and 2 (N=661), Stanford-Binet Intelligence Scale (Test Editorial Offices: Iowa City, Iowa) at ages 3 (N=611) and 4 (N=609), and the WISC-R (Wechsler Intelligence Scale for Children - Revised, Psychological Corporation, New York) at age 7 (N=606). Academic achievement was assessed by a teacher report of school performance for reading and math (Manual for the Teacher's Report Form and Teacher Version of the Child Behavior Profile, Department of Psychiatry, University of Vermont, Burlington) at ages 7 (N=425) and 9 (N=210). A multivariate analysis was carried out to estimate the additive genetic, common environmental, and individual environmental effects for the associations between the variables. The results from the most parsimonious models indicate the additive genetic component present at age 1 affects IQ up to age 4, accounting for 14% to 53% of the variance. A second genetic component present at age 3 affects IQ and academic achievement through age 9, accounting for 16% to 83% the variance. A third genetic component comes in at age 7 affection only academic achievement at ages 7 and 9, accounting for 14% to 55% of the variance. Common environment has little or no effect after age 2. Individual environment is best represented by age specific factors. Most, if not all, of the covariation between IQ and academic achievement can be accounted for by genetic affects.
Address: Campus Box 447 Boulder, CO 80309-0447 phone 303-492-1235 fax 303-, 492-8063 e-mail marcie.chambers@colorado.edu
1Institute for Behavioral Genetics, University of Colorado, Boulder, CO 80309. 2Supported by NIH Grants HD10333, HD18426, and MH43899, and from the John D. and Catherine T. MacArthur Foundation.