HELEN FISHER: Monogamy is natural. Adultery is natural too. Neither are part of the supernatural but I don't think people really understand monogamy. "Mono" means one and "gamy" means spouse. One spouse. Polygyny. "Poly" means many. "Gyny" means women. Many women. We are an animal that forms pair bonds. We are basically mono-gamous, monogamous. We're also adulterous. I think we've evolved what I call a dual human reproductive strategy. A tremendous drive to fall in love and pair up and rear our children as a team of two. And a predisposition, some people, of restlessness. An inclination to be adulterous. And we tend to be an animal that, a creature, that forms a pair bond for a period of time and breaks that pair bond and forms a new pair bond. Serial monogamy and clandestine adultery. So when did monogamy evolve? I think it evolved over 4.4 million years ago when our ancestors were coming down out of the trees. Ninety-seven percent of mammals do not pair up to rear their young. Elephants couldn't be bothered. Giraffes couldn't be bothered. Gorillas form a harem. People form pair bonds. Everywhere in the world the vast majority of people have one partner at a time.

Even in societies where the man can have a harem, polygyny, only about five to ten percent of men actually get enough cows or goats or money or education or some other sort of status to win a group of women. A woman will not be the second wife of a poor man. Only if the prerequisites outweigh the costs. I think human pair bonding evolved millions of years ago along with brain circuitry for romantic love and for deep attachment to a partner. I think it evolved for an ecological reason. Our ancestors were forced down from the trees by 8, 7, 6 million years ago, they had to begin to walk on two legs over very dangerous open grasslands. And at that point they began to stand up on two feet instead of four to carry weapons and to carry tools and to carry food back to a place where they could eat unmolested by predators. And with the beginning of walking on two legs instead of four, females began to have to carry their babies in their arms instead of on their backs. And if I were to give every woman in the world a 20 pound bowling ball to carry around for the next four years and also try to carry sticks and stones and collect fruit and vegetables and run from lions, et cetera, they too would look around for a mate.

So by four million years ago in order to survive females began to need to form a pair bond at least long enough to help raise a child through infancy. I don't see how in these open grasslands a male could have really protected a harem of females not only from wild animals but from other males. So pair bonding became essential to females and suitable to males and humanity went over what I call the monogamy threshold and we began to evolve this drive to fall in love and form a pair bond and rear our children as a team, a hallmark of the human animal today.