The number of mostly white but a little-bit-black young people -- the kind who cause confusion for affirmative action classification schemes -- is growing as interracial marriage becomes more popular. On the other hand, as Shriver's data shows, there aren't yet all that many adults who fall genetically in the "gray zone" between the races. Perhaps at present the "one drop" rule, for all its theoretical folly, still is indeed good enough for government work -- assuming that government work should include racial preferences, which are now illegal in California.
The admixture rates vary by region. The African-American populations with the highest average numbers of white ancestors found so far are those in California and Seattle. They average a little over one-quarter European ancestry.
In contrast, according to a recent article published by Shriver's team in the American Journal of Physical Anthropology, the Gullahs of the long-isolated Sea Islands of South Carolina and Georgia, who are famous for speaking a pleasantly African-sounding dialect, are only 3-4 percent white.
In the rest of the rural South, African-Americans tend to be not as black as the Gullahs, but still blacker than the national average. Shriver's team found that the white admixture percentage in four Lowland farm counties in South Carolina was 12 percent.
Cities, whether Northern or Southern, tend to be about average. In terms of white ancestry among African-Americans, New York is a little above the mean, while Philadelphia is a little below. Jackson, Miss., is near the norm.
The African-Americans of New Orleans average 22 percent white. This fairly high number reflects the influence of Spanish and French mores in Louisiana. Latin cultures have no "one drop" rule, so intermarriage was somewhat more socially acceptable there.
Advocates of the growing popular idea that race is merely a "social construct" with no biological reality point to the artificiality of the "one drop" rule as evidence for their view. Yet, it's possible that the "one drop" rule itself helped to construct the genetic reality that Shriver has uncovered.
Latin cultures, which lack the one drop rule, create more evenly blended populations, as Shriver has helped document among Mexican-Americans. He and his colleagues found that Hispanics in certain New Mexico and Colorado locales averaged 58 percent white ancestry, 39 percent New World Indian, and three percent African.
In contrast to the "bimodal distribution" of blacks and whites in America, Mexican-Americans clustered around their average admixture level of 58 percent European.
For centuries, however, American whites defined anyone with visible black ancestry as ineligible to marry a white. (It wasn't until 1967 that the Supreme Court overturned the "anti-miscegenation" laws that were then still in force in 19 states.) This meant that mixed race people could seldom marry white people.
Unless, that is, they were white-looking enough to pass for white, and were willing to pull up their roots and move to a different part of the country where they could assume a white identity. This happened not infrequently in American history. For instance, one of the slave Sally Hemmings' one-eighth black sons (who, according to geneticists, was fathered by either Thomas Jefferson or one of his relations) moved to Madison, Wis., after he was freed and founded a family of socially identified whites. Nonetheless, Shriver's data suggests that well over 90 percent of the African genes in Americans are still found in people who call themselves black.
Over the generations, mixed-race lineages would tend to either pass into the white population and become more white with each generation's marriage to a white person, or stay in the African-American population. If the latter, the families would normally become more genetically African over time as their offspring married African-Americans.
Thus, the "one drop" rule helped make African-Americans and European-Americans into two social groups whose members -- despite sometimes being highly varied in ancestry -- are perhaps more distinct on average in their family trees than the arbitrariness of the "one drop" would lead you to initially assume.