In 1979 University of Minnesota psychologist Thomas J. Bouchard studied a pair of twins, Jim Lewis and Jim Springer, who had been separated at birth. Here’s what he found on interviewing them at age 39:

• Both men had had first wives named Linda, divorced them and married women named Betty.
• Lewis named his first son James Alan; Springer named his James Allan.
• Both named their dogs Toy.
• Both had worked as gas station attendants and for the same hamburger chain.
• They drove the same type of car and bought the same brands of cigarettes and beer.
• They regularly took annual vacations at the same Florida resort.
• Both disliked baseball but enjoyed stock-car racing and woodwork.
• Both gained and lost weight at the same age, bit their fingernails compulsively and had had a minor heart attack.
• Both suffered from migraines.

“Our findings continue to suggest a very strong genetic influence on almost all medical and psychological traits,” Bouchard said. After an extensive study of separated twins, he concluded that shyness, political conservatism, dedication to hard work, orderliness, intimacy, extroversion, conformity, and a number of other social traits are largely heritable.