To arrive at these numbers, researchers pored over the genetic records of 145,000 people who submitted a cheek swab for testing to 23andme, a private company that provides ancestry-related genetic reports. The researchers examined the genetic recordsof people of self-described European, African and Latino descent to find the genetic traces left by relatives long-since deceased.
In order to hit that one percent threshold above, for instance, you'd have to have an African relative no further back than seven generations -- in other words, a great-great-great-great-great grandparent. And as you might expect, there are some fascinating differences in our genetic admixture at the state level. Southern whites are considerably more likely to have African ancestry than whites from other regions: "European Americans with African ancestry comprise as much as 12% of European Americans from Louisiana and South Carolina and about 1 in 10 individuals in other parts of the South," the authors found.
That variation makes up part of the genetic inheritance of slavery. As Jenée Desmond-Harris notes over at Vox, the study finds that present-day African-Americans are far more likely to have a European male ancestor (19 percent) than a European female one (5 percent). "That, of course, reflects what historians know about white slave owners raping enslaved women who descended from Africa," she writes.
Indeed, the average self-described African-American has about 24 percent European ancestry, according to the study, indicating that descriptors like "black" and "white" mean a lot less from a biological standpoint than they do from a cultural one. To dig deeper into this, the authors plotted respondents' proportion of African ancestry against their likelihood of calling themselves African American.
What they found was that people who were 15 percent African or less generally didn't describe themselves as African-American, while those who were 50 percent African or greater almost universally did. But in between there was a considerable amount of variation. Those who were about one quarter African were just as likely as not to call themselves African-American.
It'll be interesting to see how these proportions shift in the coming decades. In 1980, for instance, 6.7 percent of new marriages were between different-race spouses. By 2010, that share had risen to 15.1 percent. And as demographer William Frey notes, "nearly three in 10 new black marriages are multiracial, with most of them to white spouses." This is especially significant given that as recently as 1967 -- within living memory for many Americans -- interracial marriages were outlawed in 16 states.