The genealogy craze in America.

New technologies often have unforeseeable consequences. Michael Faraday could not have anticipated the rise of the electric guitar and its effects on our culture, nor did the inventors of the laser realize they had laid the ground for a thriving industry of tattoo removal. And it is safe to say that Watson and Crick could not have foreseen a day when an analysis of Oprah Winfrey's DNA would tell her that she was descended from the Kpelle people of the Liberian rainforest. "I feel empowered by this," she said upon hearing the news, overcoming her disappointment that her ancestors were not Zulu warriors.

A fascination with ancestry has long been part of the human condition, from the "begat's" of the Bible to the Roots miniseries and the restoration of Ellis Island. But with the advent of the Internet and genomic technology, genealogy has entered a new age. The past year has served up a series of high-profile revelations. The news that Barack Obama's ancestors owned slaves was a bit more surprising than the news that Strom Thurmond's did, but it was more surprising still to be told that among the Thurmond family's slaves were the ancestors of Al Sharpton. And Henry Louis Gates Jr., the host of the fascinating PBS series African American Lives, which explored the family trees of six prominent African Americans, was astounded to learn that half of his own ancestry was European, including Irish kinsmen on his father's side and two Jewish women on his mother's.

Few of us can expect that a search for ancestors will bring us an inheritance, a title, or a coat of arms: the rewards of genealogy are mostly psychological. As Winfrey put it, "Knowing your family history is knowing your worth." The sentiment, though, is dubious--not just on moral grounds but on biological ones. A closer look at the human drive to know one's family tree uncovers a number of tensions between our intuitions of kinship and the facts of kinship. Some of those facts show that the findings of the new genealogy should not have been surprising at all. And others, tacitly appreciated for millennia, have recently been neglected to our peril.

For all its fascination, kinship is a surprisingly neglected topic in the behavioral sciences. A Martian reading a textbook in psychology would get no inkling that human beings treated their relatives any differently from strangers. Many social scientists have gone so far as to claim that kinship is a social construction with no connection to biology. But assuming the creationists are wrong and humans are products of evolution, it would be surprising if our species entirely escaped the powerful forces that shape organisms' behavior toward their kin. Genetics and evolutionary theory predict that the biology of kinship should have biased our thoughts and emotions about relatives in several ways.

The first is the simple fact that blood relatives are likely to share genes. To the extent that minds are shaped by genomes, relatives are likely to be of like minds. Close relatives, whether raised together or apart, have been found to be correlated in intelligence, personality, tastes, and vices. The discovery of an ancestor is thus felt to reflect on the descendant, who may feel he has an explanation for the kind of person he is, and who can claim to have a dose of the ancestor's praiseworthy traits. A promotional spot for Coca-Cola in African American Lives juxtaposes footage of African Americans with images of traditional Africans and says, "She has her great-great-great-grandmother's eye for adornment. He is fit and agile, like his forefathers."