"Sex and War" Examines Primitive Instinct For Both

They are primal urges, sex and violence, but they seem like polar opposites. They define almost every species, but humans seem capable of keeping them apart. Or maybe not. In their new book, "Sex and War," research biologist Malcolm Potts and journalist Thomas Hayden (a former NEWSWEEK writer) probe the biological basis for the two acts and argue that, in fact, they're more closely connected than we might think.

The Idea: As humans evolved from lower primates into small clans of hunters and gatherers, violent behavior grew indelibly linked with sex and reproduction. Simply put: those who did lots of the former got to have more of the latter. It's why, even today, football players tend to get more dates than math nerds.

The Evidence: Humans are the rare mammals that kill their own kind. Scientists trace this behavior back to our closest relatives, chimpanzees, which have been observed in the wild killing rival families and even eating them. Understanding the competition for resources— reason, in other words—turns out to be our Achilles' heel. It's why pigs and dogs, which are just as interested in sex as we are, don't kill each other for it.

The Conclusion: How does the cycle end? Women are the key. In cultures where women have power over how many children they have, Potts and Hayden note, the birthrate always falls. Over time, as the competition to survive goes down, our prospects for peace go up.