The battle of the sexes in the brain Couples come into conflict when one of the partners cheats. In fruit flies, when females mate with other males, it makes the paternity of her offspring uncertain. In response, males produce chemicals that act on the female brain and decrease her sexual interest - a chastity belt for the brain. The womans brain, on the other hand, "learns" to resist this chemical. Like an arms race, men and women continue to develop strategies to get their way. Here we study how this arms race has turned the brain into a complex organ.