Humans and chimps, our closest relatives, share a curious trait: We organize to kill members of our own species.

A new book, *Sex and War, *delves into how the most intelligent apes on Earth, essentially alone in the animal kingdom, evolved the ability to organize for extreme violence.

UC Berkeley obstetrician, Malcolm Potts and science writer Thomas Hayden take a wide-ranging look at the many places that biology intersects with war. But the most fascinating parts of the book look at how modern technology has interacted with our Stone Age brains' risk calculators to produce the brutality and aggression of the world today.

In this Wired.com interview with Hayden and Potts, they talk about the evolutionary adaptation that allows us to kill our enemies, how chimps and bonobos inform our knowledge of human nature, and why the most destructive weapon might be a hormone, not a bomb.

Wired.com: Why did you write this book? Why sex and war as topics?

*

Sex and War* co-author Thomas Hayden: Let me tell you the why from two different perspectives. For me personally, the why goes back to the beginning of the Iraq war in 2003. I was a correspondent at a national news magazine (U.S. News & World Report) at the time and the war was the big story. As a science reporter, I was trying to understand the big story of the day through the lens of science.

I was struck by how big a factor the desire for revenge for 9/11 seemed to be. I was struck by the momentum, the emotional momentum, in the rush to war. It seemed once we'd been talking about war for a while, it almost became inevitable, despite lots of logical arguments against going to war. I wanted to understand why that was.

In the evolutionary psychology literature, you see that those are evolved predispositions. Those are behaviors that we see not just in our own times and in hunter-gatherer people, but, in fact, there are direct correlates we see in chimpanzees.

Wired.com: Why is it important that chimps also kill each other? What are we supposed to take from the presence of similar, violent behavior in chimps?

__

Hayden:__ The idea is very clearly established that humans and chimps are very close evolutionary cousins. We descended from a common ancestor seven million years ago. So the idea is that if there are other behaviors shared with humans and chimps, there is something that evolved in the common ancestor.

In the 1970s, Jane Goodall working at Gombe Stream National Park in Tanzania, observed what came to be known as a chimpanzee war. This behavior has been documented several times by several different people. Chimpanzees are territorial and live in troops of male relatives, dominated by an alpha male. They spend a lot of their time foraging for food but the males also spend quite a bit of time patrolling their territory.

And on occasion when they are patrolling the borders and come across someone from a neighboring troop, if they have overwhelming force — four, five or six chimps attacking one — they will launch an attack on that chimpanzee and beat and bite and rip and tear the neighboring chimpanzee, killing or leaving it for dead.



In the case of the chimpanzees that Goodall first observed, one troop completely annihilated and/or absorbed a neighboring troop, essentially conquering them.

The pattern is what we see in our warfare even today. It depends on surprise and on overwhelming force. The correlate for that would be the shock and awe of the invasion of Iraq. It also depends on a critical evolutionary innovation that allows war to happen.

This behavior of intentionally gathering together and going out to kill members of our own species is an extremely rare behavior. Humans do it. Chimps do it. There is some evidence that wolves and hyenas do it. But it's pretty much a human and chimp innovation.

You have a very intelligent animal and a social animal. And when you're a social animal, all of the evolutionary pressures are toward living in a group. There are hierarchies. There are mechanisms for resolving disputes in nonlethal ways. That can all be summed up under empathy. But humans and chimpanzees, when they are fighting an out-group, have the ability to turn off the empathy. By turning that off, you dehumanize the enemy or dechimpize the enemy.

Wired.com: Are there records of chimps who are more or less warlike? If so, what are the parameters that change that behavior?

Sex and War co-author Malcolm Potts: Probably competition for resources and density. It's a good question and we don't really have enough information to answer it in a totally scientific way. Most of the places chimps live are constrained because there are farmers all around them.

When Goodall first observed these things, [people] said it was unnatural because they fed them bananas. Whenever there has been systematic study of chimps, there have always been episodes of same-species killing.

Wired.com: You've studied the demographics of war. What types of societies are more likely to go to war?

Potts: I think there are several things going on. If you have a lot of competition for resources — fig trees or oil — then there is a higher change. But a more subtle thing is that when you have a rapidly growing population and you have a lot of young people in relation to older people — young men in relation to older men — it makes it easier for conflict to break out.

Obviously in a complicated set of social behaviors ... but it looks like this is a factor that counts for a proportion of the risk of having a war. We feel it's one of the factors that is open to variation. It's something we can deal with. We can slow population growth if we do it in a respectful way.



Wired.com: You both talked about how men are really the drivers of war. Why is that?

Potts: Because of the asymmetry in the investment [men and women] make in the next generation, beginning with eggs being bigger than sperm. When you're a mammal, women can only have a limited number of children. Their sexual agenda is to be as selective and to get support from that mate. Whereas the males amongst chimpanzees and to some extent among human beings, the more sexual partners they can get, the more likely they pass their genes to next generation. So the males are competing.

Most peoples, not the number of people, but the number of cultures, are monogamous. Men are intrinsically risk-taking and are less selective in their sexual partners and once you get this team aggression in a primate, a new set of things kick in. You add all those things together and you've got a pretty fearsome male animal. That's why I call testosterone the perfect weapon of mass destruction.

Wired.com: How has technology changed the nature of this warring behavior, its biology?

Hayden: That's a really important question. The difference between a band of chimpanzees attacking with teeth and nails versus a predator drone firing hellfire missiles from beyond the horizon is very significant.

Let me point to two things. One is the development of weapons that can kill from a distance. And it probably began by throwing a rock or a sharp stick. And then it was a sharp stick with something sharp on the end of it. In this purest form of battle, you have a small band of males ganging up on an enemy. When even four or five chimpanzees attack one chimp, there is a pretty high risk of being injured. Just like in a fistfight, it's rare you come out of a fist fight without a broken nose or a cracked knuckle. There's a pretty high bar for making that attack because there is a risk of death.

But as a soon you can kill from a distance, that calculus begins to shift and that barrier begins to drop. You take it up a notch to a bow-and-arrow, and maybe that you can shoot from behind a tree, and you can kill without being detected yourself. Your risk goes down to near zero. So, what happens, as you increase the sophistication of the killing technology and your ability to kill from a distance, you decrease the barriers to launching an attack, so you increase the amount of war and violence.

That's something you see through human history: The most warlike cultures and societies are the ones that have developed simple distance killing techniques ... Bows, slings, that sort of thing.

At the extreme other end of technology, you've got nuclear weapons and other weapons of mass destruction where that calculus is turned on its head. You have the potential of using your own weapon, but you also have what we've called for decades now, mutually assured destruction. The risk is so severe that it reverts back to the earlier calculus. If I use this weapon, it could come back on me.

So, at the beginning of human technical innovation, you have simple technologies enabling warfare and making it more common and at the far end of it, you have technologies that flip that technology back to the early stages.

Wired.com: Could you point to any other specific technological leaps that really changed the nature of warfare?

Hayden: Here's a really important one, maybe the most important one today. And that is the way in which technology enables terrorism. I want to say this carefully and this is a really important point any time you are talking about the evolution of human behavior. It's very clear that we are evolved animals and there are behavioral dispositions. But to say that something has evolved doesn't put a value statement to it. It doesn't say it's a good or bad or necessary behavior. We're very complex animals, so there are predispositions that tilt us towards distrust or hatred of outsiders, love and compassion for members of our in-group.

The balance of those different traits is such that perhaps all men have the ability to be warriors. We have the evolved traits necessary to turn off that empathy. But that doesn't mean there isn't any free choice and there is a lot of environmental circumstance. Nature provides the possibilities and nurture helps shape what actually happens.

So, when it comes to terrorism, it only works because of technology, because a small number of people, almost always men, can use technology for leverage. Nineteen terrorists armed with sticks and stones could do very little to affect the United States of America. But 19 terrorists armed with jet fuel-laden aircraft ... The technology pushed their destructive capability way beyond where it would have been. Nineteen men against 300 million people. We would have never known they existed if they hadn't leveraged technology.

Technology has done many wonderful things for humankind through the years, but it also has been a central part of war. The technology of a time really defines the warfare of a time.

Wired.com: Does the study of the bonobos, another close primate relative of humans who are noted for their peaceful behavior, add anything to the discussion of sex and war?

Hayden: I think it does. Chimpanzees and bonobos are sort of a Rorschach test for humanity. Do you see us as warring, meat eaters or vegetarian peace lovers who apparently solve all their problems by having sex?

My very loving view of the human condition has room for the chimpanzees and the bonobos. Thank goodness we have both species ... If we just had chimpanzees, we might not be quite as hopeful. With the bonobos, we find a great deal of diversity of behavior. I think humans have the capacity for love and peaceful coexistence.

The really hopeful thing of looking at war from the perspective of evolution is recognizing that war is built up from a set of evolved predispositions, but that doesn't make war inevitable. Yes, it is inherent, but it's not necessary and we can start looking at things that we can do in social policy that make war less likely and less brutal.

You can look at it as trying to figure out what we can do and how we can shape our world so that our bonobo comes out more than our chimpanzee nature. And when you get right down to it, who wouldn't rather be a bonobo?

Image: flickr/blueforce4116

See Also:

WiSci 2.0: Alexis Madrigal's Twitter , Google Reader feed, and webpage; Wired Science on Facebook.