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

Why is all this variety possible? That's not a question that can be answered by looking at the political, economic, or cultural aspects of human kinds, as important as those aspects are. The issue is not what human kinds are in the world, but what they are in the mind - not how we tell Tamils and Seventh-Day Adventists and fans of Manchester United from their fellow human beings, but why we want to.

After all, other creatures get along fine without dividing themselves into such tribes. With one important exception, for instance, humanity's close relative the chimpanzee goes through life quite well solving only problems about Everychimp and problems about My Friend and Cousin, the one with the long face and the limp - chimps in general and individuals. Yet a human who went through life like that would not know what "our kind" of food is, or enjoy "our kind" of music, or know what "we do" when someone dies.

Such a person, lacking any sense of family tradition, religious history, patriotism, or cultural pride, would not live a fully human life. Human-kind thinking is an absolute requirement for being human.

Which brings up the dark side: people are killed for nothing more than their membership in the "wrong" tribes. Many times in the past few years, young men who were polite and thoughtful to members of their own sort, who loved their mothers and listened to their fathers and cared for their children, set out to kill other people's mothers and fathers and children without a qualm - in New York City on September 11, 2001; in Beslan, Russia, on September 1, 2004; in Nazi-ruled Europe in the 1930s and 1940s. More examples could fill every page of this book. Throughout history, killers have worked with zeal because they believed their victims were not of the same kind as the people they cared about, the people who mattered. No wonder people both cherish and fear the power of tribal thinking. No wonder they want to understand it.

Many people today find themselves struggling, again and again, with a difficult question: Who is "our side"? In the days after the September 11 attacks, the prime minister of Italy spoke of the superiority of Christian civilization and was immediately denounced by his allies; two religious leaders in the United States tried to claim that abortionists and civil libertarians did not belong in the new alliance against terror because they made God angry. The president of the United States, though he was a political ally, rebuked the clergymen. The new anti-terror coalition of nations could not make war on Islam because they had millions of Muslim citizens. Was it, then, a coalition of democracies? The most important frontline ally in the ensuing war was Pakistan, a dictatorship. Obviously, sides in the current conflict were chosen; they aren't a matter of "natural" or inevitable divisions. Today it is clear as never before that human kinds - those categories we use to explain human acts on every scale, from a morning walk ("Why were those men wearing turbans?") to all of history ("Is war inevitable?") - don't depend on what people are, but on what people believe.

That's the difference between human tribes and the boundaries animals observe. Many creatures, from mice and pigeons to lions and dolphins, know a member of "our group" from a stranger. All these creatures have been well studied in the past few decades, and the pattern is clear: fights within the group are limited and tend not to get out of hand, but fights between groups end in deaths. This in-group restraint, as the psychiatrist and author Jonathan Shay has said, gave rise to the widely believed myth that animals don't kill their own kind. In fact many animals - lions, gorillas, chimpanzees, hyenas - are happy to kill their own. The victim just can't be a member of their little band.

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