Just the existence of such genetic differences between races, proclaimed the author of the Half Sigma blog, a 40-year-old software developer, means "the egalitarian theory," that all races are created equal, "is proven false."

Although few of the bits of human genetic code that vary between individuals have yet been tied to physical or behavioral traits, scientists have found that roughly 10 percent of them are more common in certain continental groups, and can be used to distinguish people of different races. They say that studying the differences, which arose during the tens of thousands of years human populations evolved on separate continents following their ancestors' dispersal from humanity's birthplace in East Africa is crucial to mapping the genetic basis for disease.

Many geneticists are loath to discuss the social implications of their findings. Still, some acknowledge that as their data and methods are extended to non-medical traits, the field is at what one leading researcher recently called "a very delicate time and a dangerous time."

"There are clear differences between people of different continental ancestries," said Marcus Feldman, a professor of biological sciences at Stanford University. "It's not there yet for things like IQ, but I can see it coming. And it has the potential to spark a new era of racism if we do not start explaining it better." Feldman said any finding on intelligence was likely to be exceedingly hard to pin down. But given that some may emerge, he wants to create "ready response teams" of geneticists to put such socially fraught discoveries in perspective.

Some fear that the authority DNA has earned through its use in freeing wrongly convicted inmates, preventing disease and reconstructing family ties, scientists warn, leads people to wrongly elevate genetics over other explanations for differences between groups.

"I've spent the last 10 years of my life researching how much genetic variability there is between populations," said Dr. David Altshuler, director of the Program in Medical and Population Genetics at the Broad Institute in Cambridge, Massachusetts. "But living in America, it is so clear that the economic and social and educational differences have so much more influence than genes. People just somehow fixate on genetics, even if the influence is very small." But on Half Sigma and elsewhere, the conversation is already flashing forward to what might happen if genetically encoded racial differences in socially desirable - or undesirable - traits are identified.

"If I were to believe the 'facts' in this post, what should I do?" one reader responded. "Should I advocate discrimination against blacks because they are less smart? Should I not hire them to my company because odds are I could find a smarter white person? Stop trying to prove that one group of people are genetically inferior to your group. Just stop."