Helen E. Fisher, PhD biological anthropologist, is a Research Professor in the Department of Anthropology at Rutgers University. Fisher maintains that humans have evolved three core brain systems for mating and reproduction. “Love can start off with any of these three feelings,” Fisher maintains. “Some people fall head over heels in love, but the sex drive evolved to encourage you to seek a range of partners; romantic love evolved to enable you to focus your mating energy on just one at a time; and attachment evolved to enable you to feel deep union to this person long enough to rear your infants as a team.” What happens when you fall in love?