Nature 336, 767 - 769 (29 December 1988); doi:10.1038/336767a0

Genetic influence on general mental ability increases between infancy and middle childhood

D. W. Fulker^*, J. C. DeFries^* & Robert Plomin^†

^*Institute for Behavioural Genetics, University of Colorado, Boulder, Colorado 80309–0447, USA
^†College of Human Development, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

Adoption studies can provide direct evidence for the independent effects of family environment and heredity that are always confounded in intact nuclear families^1–3. When children are separated from their biological mothers shortly after birth and placed non-selectively in adoptive homes, adoptive-parent/adopted-child resemblance can be ascribed to cultural transmission, whereas biological-parent/adopted-child similarities are due to heritable factors. Furthermore, a longitudinal adoption study facilitates examination of changes in these two main sources of variation during development. The Colorado Adoption Project is the first large-scale longitudinal adoption study of behavioural development and was initiated in 1975⁴. Data were collected from biological parents of 245 adopted children, the adoptive parents and parents of 245 matched nonadopted children. The children have subsequently been tested at 1, 2, 3 and 4 years of age, and at the end of their first year in primary school (average age, 7.4 years). The number of subjects tested is now adequate for analysis of data over 7 years. The results provide conclusive evidence for increasing heritable variation of general mental ability, ranging from 9% at 1 year of age to 36% at 7 years.

References

1.	Cavalli-Sforza, L. L. in Developmental Human Behavior Genetics (eds Schaie, K. W., Anderson, V. E., McClearn, G. E. & Money, J.) (Lexington, 1975).
2.	Kamin, L. In The Intelligence Controversy (eds Eysenck, H. J. & Kamin, L.) (Wiley-Interscience, New York, 1981).
3.	Lewontin, R. C. A. Rev. Genet. 9, 387−405 (1975). | Article | ChemPort |
4.	Plomin, R. & DeFries, J. C. in Origins of Individual Differences in Infancy: The Colorado Adoption Project (Academic, Orlando, 1985).
5.	Skodak, M. & Skeels, H. M. J. Genet. Psychol. 75, 85−125 (1949). | PubMed | ChemPort |
6.	DeFries, J. C., Plomin, R., Vandenberg, S. G. & Kuse, A. R. Intelligence 5, 245−277 (1981). | Article |
7.	Bayley, N. in Manual for the Bayley Scales of Infant Development (Psychological Corporation, New York, 1969).
8.	Terman, L. M. & Merrill, M. A. in Stanford-Binet Intelligence Scale: 1972 Norms Edition (Houghton Mifflin, Boston, 1973).
9.	Wechsler, D. L. in Examiner's Manual: Wechsler Intelligence Scale for Children-Revised (Psychological Corporation, New York, 1974).
10.	Fulker, D. W. & DeFries, J. C. Br. J. math. & stat. Psychol. 36, 175−188 (1983). | PubMed |
11.	Carey, G. Am. J. hum. Genet. 39, 775−786 (1986). | PubMed | ChemPort |
12.	McGue, M., Wette, R. & Rao, D. C. Genet. Epidemiol. 1, 255−269 (1984). | Article | PubMed | ChemPort |
13.	Wilson, R. C. Child Dev. 54, 298−316 (1983). | PubMed | ChemPort |
14.	Eaves, L. J., Long, J. & Heath, A. C. Behav. Genet. 16, 143−162 (1986). | Article | PubMed | ChemPort |