The Minnesota Study of Twins Reared Apart

Launched in 1979 under the direction of the psychologist Thomas Bouchard at the University of Minnesota, the Minnesota Study of Twins Reared Apart recruited identical and fraternal twin pairs who had been separated in infancy or early childhood and raised in different families, eventually bringing more than one hundred sets of reared-apart twins and triplets to Minnesota for roughly a week of intensive psychological and physiological testing each, covering measures of personality, intelligence, occupational and leisure interests, and social attitudes. Because identical twins reared apart share essentially all of their genes but grew up in different environments, while fraternal twins reared apart share on average only about half their genes under similarly different environments, comparing the two groups allowed the researchers to estimate the relative contributions of genetic and environmental influence on the traits measured. The study's central and most widely reported finding was that identical twins reared apart were, on most personality, temperament, interest and attitude measures, about as similar to one another as identical twins who had been raised together in the same household, suggesting a substantial genetic contribution to individual differences in these traits. The Minnesota study became one of the most influential behavioral genetics studies of the twentieth century, cited widely in debates over the relative roles of nature and nurture in shaping personality, though its methodology and some of its more striking anecdotal case findings have also drawn considerable scholarly scrutiny and criticism.