Like many identical twins reared apart, Jim Lewis and Jim Springer found they had been leading eerily similar lives. Separated four weeks after birth in 1940, the Jim twins grew up 45 miles apart in Ohio and were reunited in 1979. Eventually they discovered that both drove the same model blue Chevrolet, chain-smoked Salems, chewed their fingernails and owned dogs named Toy. Each had spent a good deal of time vacationing at the same three-block strip of beach in Florida. More important, when tested for such personality traits as flexibility, self-control and sociability, the twins responded almost exactly alike.
The two Jims were the first of 348 pairs of twins studied at the University of Minnesota, home of the Minnesota Center for Twin and Adoption Research. Much of the investigation concerns the obvious question raised by siblings like Springer and Lewis: How much of any individual's personality is due to heredity? The center's answer: about half.
The project, summed up in a scholarly paper that has been submitted to the Journal of Personality and Social Psychology, is considered the most comprehensive of its kind. The Minnesota researchers report the results of six-day tests of their subjects, including 44 pairs of identical twins who were brought up apart. Well-being, alienation, aggression and the shunning of risk or danger were found to owe as much or more to nature as to nurture. Of eleven key traits or clusters of traits analyzed in the study, researchers estimated that a high of 61% of what they call "social potency" (a tendency toward leadership or dominance) is inherited, while "social closeness" (the need for intimacy, comfort and help) was lowest, at 33%.
The study finds that even a penchant for conservatism seems to have a genetic base. One of the eleven traits, traditionalism (respect for authority, rules, standards and high morals), was discovered to be 60% inherited. Among other traits listed at more than 50% were vulnerability or resistance to stress, dedication to hard work and achievement and the capacity for being caught up in imaginative experiences.
The director of the study, Thomas Bouchard, cautions that the numbers so far may not be strictly accurate. "In general," he says, "the degree of genetic influence tends to be around 50%." Attributing the 28-point gap between potency and closeness to possible sampling errors, he predicted that "social potency will drop and social closeness will creep up."
All the twins took several personality tests, answering more than 15,000 questions on subjects ranging from personal interests and values to phobias, aesthetic judgment and television and reading habits. Twins reared separately also took medical exams and intelligence tests and were queried on life history and stresses. Not all pairs matched up as well as the two Jims. California Twins Ann Blandin and Barbara Parker, 40, showed only minor similarities. Said Blandin: "Bouchard said we were the most different set of twins in the study."