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...