Excerpt from Us and Them by David Berreby, plus links to reviews, author biography & more

Animals, though, don't make decisions about who is "in" and who is "out." A dog guards her puppies because they are kin, and members of her human family because they are friends. But no dog quits her humans because they have converted to Catholicism or put a peace sign on the lawn.

People can and will make that sort of change, because people, unlike animals, make choices based on signs - crosses, uniforms, peace signs, oaths, and other indicators of a particular human kind. Animals have kin and animals have friends, but only human beings trust symbols to tell who is kin and who is a friend.

One August night in New York City in 1997, for example, the crucial symbol was a tiny piece of metal. A white cop beat up a black man he had arrested. Later, both men were in the bathroom of the police station, where the officer spotted a tiny crucifix that his victim wore around his neck. That was enough to make the cop put away one map of human kinds and take up another - instead of police against suspect, or white against black, he now saw two fellow Christians. The officer said he too believed in Jesus, and apologized.

In Sovu, Rwanda the symbol was a bit of cloth. That day, Tutsi refugees sought escape from bands of Hutus in Sovu's convent. The mother superior, Sister Gertrude, called in the Hutu militia. Hundreds of the Tutsi were shot, hacked, or burned to death. But Sister Gertrude did not turn over the convent's Tutsi nuns. Their veils protected them. Seeing this, a nineteen-year-old woman named Aline, the niece of a nun, begged for a veil. Sister Gertrude refused.

Seven years later, she was convicted in Belgium of war crimes. Among the witnesses was the murdered niece's mother. "My daughter was killed because of a little piece of cloth," she said. If humans are, as the neuroscientist Terence Deacon puts it, the "symbolic species," then human kinds are among those features that reveal our uniqueness. A symbolic strip of cloth - its presence saving you from a pack of rampaging killers, its absence marking you as the kind to kill - is something only Homo sapiens creates.

But a symbol, like that nun's veil, is meaningless unless it is understood. If the murderers had thought it was just a bit of cloth, for example, they would have killed all the Tutsi who wore it. Any activity that depends on symbols can't be understood without taking into account the human minds that use those symbols. Imagine trying to get by in Kinyarwanda, the language shared by Hutu and Tutsi, by treating it only as a system of sounds - wavelengths and acoustic properties. You wouldn't get far until you accepted that these sounds meant something, and found someone to explain what those meanings were.

In the same way, human kinds can't be understood objectively, as a collection of facts about blood types, skull shapes, average ages, preferred brands, and so on. Those facts seen from the outside can never tell what the human kind means. That meaning is made inside the heads of people who believe in it.

Scientists who study a pack of macaque monkeys can predict who gets along with whom. Knowing which animals are relatives and which are allies lets an observer explain the fights and frolics very well. But "objective" knowledge of human kinds does not. Sister Gertrude was a Hutu, so you would not be surprised if she had sent all the Tutsi in the convent to death. Yet she was also a nun, so it's not a surprise that she saved fellow nuns even though they were Tutsi. She was also a Christian; it would be admirable and understandable if she had stood, on religious principles, against the killing of any human being.

Copyright © 2005 by David Berreby.  No part of this book maybe reproduced without written permission from the publisher.