Recent studies of twins have shaken the field of psychology to its foundation, revolutionizing our understanding of our own personalities. Because identical twins separated at birth share all the same genes, yet live separate lives, they offer a unique opportunity to test theories about the roles played by nature and nurture in shaping who we are.

Twins directly challenges many long-held beliefs. For instance, a series of groundbreaking studies of twins has shown that our genes play a much stronger role in shaping our identities than previously thought. Today, scientists can actually estimate what proportion of our intelligence, our personality, and our behavior is determined by inherited tendencies. Even our political orientation and our religious commitment, it turns out, are largely governed by our genes.

Twins is filled with astounding stories of identical twins who have lived entirely separate lives but have an incredible amount in common: their hobbies, their mannerisms, their taste in music, food, and clothes, their experiences in marriage and divorce, their careers, their sexuality, even the names they've given their children.