For a century, scientists have been trying to figure out which factors play the most important role in the development of the human personality. Is it nature or nurture, heredity or environment? The best information so far has come from the study of identical twins reared apart. Like the embryo clones of the future, their genes are identical because they come from the same egg and sperm. The fertilized egg splits in half shortly after conception, forming two identical embryos. (Fraternal twins come from two separate eggs and sperms and are no closer genetically than ordinary siblings.
Raised together, twins often are unusually close, sometimes developing their own private language. But even when they are reared apart, twins show amazing similarities as adults. Twins Jim Springer and Jim Lewis, separated at birth in 1939, were reunited 39 years later in a study of twins at the University of Minnesota. Both had married and divorced women named Linda, married second wives named Betty and named their oldest sons James Allan and James Alan. More coincidences: both drove the same model of blue Chevrolet, enjoyed woodworking, vacationed on the same Florida beach and had dogs named Toy.
Perhaps the superficial details of the Springers’ parallel lives were just a coincidence, but scientists who study twins like them say they have discovered fundamental personality traits that are the same no matter how the twins were reared. In the Minnesota study, 350 pairs of identical twins reared apart were put through a battery of physical and psychological tests. The results showed that such characteristics as leadership ability, imagination, vulnerability to stress and alienation were largely inherited. On the other hand, the environment the child was raised in seemed to account for a whole host of other traits, including aggression, achievement, orderliness and social closeness.
Heredity may also play a role in determining sexual orientation. In a controversial 1991 study, researchers from Northwestern and Boston universities found that if one identical male twin is gay, the other is almost three times more likely to be gay than if the twins are fraternal. Earlier this year the same scientists released a study of 147 lesbians with similar results.
While these studies do reveal tantalizing clues about how personalities are born and bred, they’re far from the whole story. Some identical twins turn out very different from their siblings. One twin in the Minnesota study, for example, grew up to be a stellar pianist even though she was adopted by a nonmusical family. Her sister, adopted by a piano teacher, never sits down at the keyboard. Genes may do the initial programming, but a lifetime of experiences, unique for each person, shape the soul.
