Fascination with twins and what twin studies tell us about human nature is universal. There is no question that behavioral and medical science advances have happened largely because twins yield a wealth of information just by being themselves. Twin research takes place at two levels: (1) Studies that are strictly for twins. Such studies examine the pros and cons of twins being separated at school, dressing alike, or sharing their friends. (2) Studies with broad implications. Psychological analyses and medical research are conducted to understand the genetic and environmental factors affecting behavior and health which can be applied to the general nontwin population.
Identical twins reared apart—the rare “elite pairs” among multiples because they cleanly separate genes and experience—have revealed genetic effects on intelligence, personality, interests, and more. One of the most striking findings from the investigators of the Minnesota Study of Twins Reared Apart is that the personality traits of reared-apart identical twins are as similar as the personality traits of reared-together identical twins. This suggests that personality similarity among family members living together is explained by their shared genes, not their shared environments.
