One of science’s so far uncrackable mysteries is the comparative impact of heredity vs. environment. An obvious experimental method would be to raise identical twins separately, but that could hardly be done with humans. So for the last 10 years University of Minnesota psychologist Thomas Bouchard, 41, has been studying twins under less than ideal, lab-controlled conditions—until, eureka, he ran into the stuff of social scientists’ dreams. Identical twin males, who had been separated by adoption at three weeks, suddenly rediscovered each other in Ohio at age 39.

Within two weeks after reading about them in the press, Dr. Bouchard had the twins in his Minneapolis lab for tests. At the outset of his investigation the psychologist said, “I think there are going to be all kinds of differences that will surprise even the twins.” But what was immediately apparent were eerie similarities that left even Bouchard “flabbergasted.”

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Curiously, both had been christened James by their adoptive parents, the Jess Lewises of Lima and the Ernest Springers of Piqua, 40 miles away. As schoolboys, both enjoyed math and carpentry—but hated spelling. Both pursued similar adult occupations: Lewis is a security guard at a steel mill, and Springer was a deputy sheriff (though he is now a clerk for a power company). Both married women named Linda, only to divorce and remarry—each a woman named Betty. Both have sons: James Alan Lewis and James Allan Springer.

The two men shared one other fact in common. As Jim Springer put it, “I always felt an emptiness.” Neither the Springers nor the Lewises ever met the 15-year-old (unwed) mother of their sons, and both couples were told that their adoptive child had a twin who died at birth. Then one day, when Jim Lewis was 16 months old, his mother visited the Miami County courthouse to settle the adoption paperwork, and an official remarked offhandedly, “They named the other little boy ‘Jim’ too.”

For 37 years that hint tugged at Mrs. Lewis, who occasionally urged her son to find out if it was true. Finally, last Thanksgiving, he agreed to search—though he says he doesn’t know why. Jim Lewis wrote the probate court, which had a record of the adoption, and contacted the Springer parents in Piqua. “I came home one day,” Lewis recounts, “and had this message to call ‘Jim Springer.’ ” When he phoned Springer, Lewis blurted out: “Are you my brother?” “Yup,” Springer replied. Four days later, last Feb. 9, Lewis drove to meet his twin for an emotional reunion.

Dr. Bouchard offered expenses and a small honorarium to get them to Minneapolis for a week of extensive physical and psychological tests. He wanted to begin as soon as possible to preclude their reminiscing together too long and thus “contaminating” the evidence. Though not the first such separated twins—the records show 19 previous sets in the U.S. among some 75 worldwide—Lewis and Springer were believed to have been apart by far the longest.