For centuries, Americans have classified themselves and their neighbors by the color of their skin.
Belief in the reality of race is at the heart of how people traditionally perceive differences in those around them, how they define themselves and even how many scientists say humanity evolved.
Today, however, a growing number of anthropologists and geneticists are convinced that the biological concept of race has become a scientific antique--like the idea that character is revealed by bumps on the head or that canals crisscross the surface of Mars.
Traditional racial differences are barely skin deep, scientists say.
Moreover, researchers have uncovered enormous genetic variation between individuals who, by traditional racial definitions, should have the most in common.
Some scientists suggest that people can be divided just as usefully into different groups based on the size of their teeth, or their ability to digest milk or resist malaria.
All are easily identifiable hereditary traits shared by large numbers of people. They are no more--and no less--significant than skin tones used to popularly delineate race.
"Anthropologists are not saying humans are the same, but race does not help in understanding how they are different," said anthropologist Leonard Lieberman of Central Michigan University.
The scientific case against race has been building quietly among population geneticists and anthropologists for more than a decade.
This month, the International Union of Anthropological and Ethnological Sciences is expected to vote on whether races really exist. The American College of Physicians this week urged its 85,000 members to drop racial labels in patient case studies because "race has little or no utility in careful medical thinking."
But even if accepted, recent scientific findings on race cannot be expected to do away with centuries of social and political policies.
Many social scientists, medical researchers and public health experts routinely make race-based comparisons of health, behavior and intelligence--even though many of them acknowledge that such conclusions may be misleading.
As a result, the creation of racial and ethnic categories for public health purposes is becoming increasingly contentious, experts say. U.S. census officials also are snarled in an effort to redefine how people can best be classified.
"No one denies the social reality of race," said anthropologist Solomon H. Katz of the University of Pennsylvania. "The question is what happens to the social reality when the biological ideas that underpin it vanish?"
"I find the term \o7 race \f7 pretty useless scientifically," said Luigi Luca Cavalli-Sforza.
The Stanford Medical School scholar, one of the world's leading geneticists, is part of a global effort to identify the thousands of genes that make up the human biological blueprint and to explore its unique genetic variations.
Cavalli-Sforza, 72, has compiled a definitive atlas of human genetic diversity. The "History and Geography of Human Genes," which draws on genetic profiles of 1,800 population groups, is the most comprehensive survey of how humans vary by heredity.
Fourteen years spent surveying the global genetic inheritance has convinced him and his colleagues that any effort to lump the variation of the species Homo sapiens into races is "futile."
"We have the impression that races are important because the surface is what we see," Cavalli-Sforza said. "Scientists have been misled this way for quite a while, until recently, when we had the means to look under the skin."
What they did not find when they looked under the skin has been just as revealing as what they did find.
Their work undermined an idea of race as old as the United States. Ever since the 18th Century, when scientists first formally codified humanity into races by skin color, "humor" or temperament, and posture, most researchers have accepted the fundamental idea that people are divided into fixed racial types, defined by predictable sets of inherited characteristics.
If this concept of race had scientific validity, researchers would expect to find clusters of significant genetic traits arranged by skin color and population group.
When scientists examined human genetic inheritance in detail, however, they found that inherited traits do not cluster and do not stay within any particular group, debunking the idea of homogeneous races.
For example, the sickle-cell trait usually is treated as a hereditary characteristic of black Africans. As a single gene, it confers resistance to malaria, although if inherited from both parents, it can lead to sickle-cell anemia. But the trait appears wherever people had to cope with prolonged exposure to malaria. It is as prevalent in parts of Greece and south Asia as in central Africa.