In early 1979, a pair of identical twin brothers who had been separated at four weeks were reunited after 39 years. Both named Jim, they discovered that they smoked the same brand of cigarettes, vacationed in the same town and both called their dog “Toy.” Struck by the story, psychologists at the University of Minnesota started studying separated twins that same year. Their efforts blossomed into the Minnesota Study of Twins Reared Apart, which ran for a quarter century, attracting world-wide fascination and antipathy.
Nancy Segal’s “Born Together—Reared Apart” is a thorough history of the project and of the 137 pairs of star-crossed twins who made it possible. Ms. Segal, a key member of the Minnesota team, focuses on the many scientific publications that emerged from the data. But along the way, readers meet leading twin researchers and a whole lot of twins—including the “Jim twins,” the “giggle twins” (who both laughed almost nonstop) and, most incredibly, Oskar, raised as a Nazi, and his identical brother, Jack, who was raised as a Jew.
The Minnesota researchers tracked down every pair they could find—and measured traits related to almost every aspect of life: health, cognition, personality, happiness, career, creativity, politics, religion, sex and much more. The Minnesota study reveals genetic effects on virtually every trait. The breakdown between nature, nurture and everything else varies from trait to trait. But Ms. Segal emphasizes the uniformity of the results—the consistent power of genes, the limited influence of parenting.
The Minnesota study’s IQ results hit a nerve years before their publication in 1990, overshadowing other controversies that might have been. Many of its findings are bipartisan shockers. Take religion, which almost everyone attributes to “socialization.” Separated-twin data show that religiosity has a strong genetic component, especially in the long run: “Parents had less influence than they thought over their children’s religious activities and interests as they approached adolescence and adulthood.” The key caveat: While genes have a big effect on how religious you are, upbringing has a big effect on the brand of religion you accept. Identical separated sisters Debbie and Sharon “both liked the rituals and formality of religious services and holidays,” even though Debbie was a Jew and Sharon was a Christian.