Successful narcissists have a whole lot of sex, which means they’re statistically likelier to have a whole lot of babies—at least compared to everyone else. This highly adaptive component of narcissism gives it a big edge over other disorders in getting passed down to the next generation.

“When you’re talking about truly pathological narcissism, it’s hard to say why it hasn’t been eradicated from the gene pool,” says Jessica Tracy, a professor of personality and social psychology at the University of British Columbia. “But when you’re talking about the more everyday kind, it’s equally hard to see how it ever would disappear since it’s so adaptive. All you have to do is replicate yourself, and narcissists are very good at doing that.”

— From The Narcissist Next Door: Understanding the Monster in Your Family, in Your Office, in Your Bed—in Your World, by Jeffrey Kluger (to be published this month by Riverhead)