Think twice before you call someone promiscuous—it could just be the way they were born. A study of identical twins separated at birth reveals that the age at which a person first has sex is determined in part by his or her genes. Innate traits such as impulsivity further the chances that a person will engage in sex earlier. Nancy Segal, the study’s leader and a psychologist at California State University, said other factors could also affect when people first have sex. The study found that women who reported being unhappy or unfulfilled had sex at an earlier age than others. Segal’s team studied 48 pairs of twins who were raised apart from each other, and 23 individual twins.