CU R RE N T D I R E CT I O NS IN P SYC H OL OGI C AL SC I EN C E
Beyond Heritability
Twin Studies in Behavioral Research
Wendy Johnson,1,2 Eric Turkheimer,3 Irving I. Gottesman,2,3,4 and Thomas J. Bouchard, Jr.2 Department of Psychology and Centre for Cognitive Ageing and Cognitive Epidemiology, University of Edinburgh; Department of Psychology, University of Minnesota–Twin Cities; 3Department of Psychology, University of Virginia; and 4 Department of Psychiatry, University of Minnesota–Twin Cities Medical School
2 1
ABSTRACT—The
heritability of human behavioral traits is now well established, due in large measure to classical twin studies. We see little need for further studies of the heritability of individual traits in behavioral science, but the twin study is far from having outlived its usefulness. The existence of pervasive familial in??uences on behavior means that selection bias is always a concern in any study of the causal effects of environmental circumstances. Twin samples continue to provide new opportunities to identify causal effects with appropriate genetic and shared environmental controls. We discuss environmental studies of discordant twin pairs and twin studies of genetic and environmental transactions in this context.
KEYWORDS—heritability; twin study; genetic and environmental in??uences; discordant twin pairs; gene–environment transactions; epigenetics
Throughout the world, perhaps 800,000 pairs of twins have been collected into more than 50 different study samples. Many of these samples would be of considerable epidemiological value even if they did not contain twins, because they are closely representative of the populations from which they have been drawn, having been recruited from birth and other systematic records. Like all experiments, these experiments of nature are not perfect: Differences between twins and singletons, differences between monozygotic (MZ) and dizygotic (DZ) twins, and methodological biases and limitations could be distorting estimates of the extent of genetic in??uence. But the sheer volume of evidence from twin studies and the corroborating evidence from adoption studies and studies of other combinations of relatives makes it unreasonable to deny the presence of genetic in??uences on behavior. And because the genotype pre-exists all behavior, these genetic in??uences have to be considered causal at some level.

下一页