While some of the influence can be chalked up to environment, research is beginning to sniff out another, more plausible culprit: pheromones. Yes, unshowered brothers and biological fathers everywhere are sweating out cues that inhibit female sexual development. It even has a name: "sweat-stifles-sexuality theory." To explain why it's an evolutionary necessity, I'll let sex and science writer Jena Pincott of Science of Sex, Love, and Babies talk.
"Over the years, a girl would inhale chemical cues in fraternal sweat — think of all those sock and armpit odors. Those chemicals would hit the hypothalamus of her brain where sex hormones are produced, and slow down the works. Puberty strikes a little later. Evolutionarily speaking, the result is that a girl could stay in the family nest longer without conflict. The risk of incest is reduced."
Ugh, wake me up in 1,000 years when evolution has rendered biological incest blockers irrelevant.