Consistent with previous anecdotal results,32 the frequency of European American individuals who carry African ancestry varies strongly by state and region of the US (Figure 3A). We estimate that a substantial fraction, at least 1.4%, of self-reported European Americans in the US carry at least 2% African ancestry. Using a less conservative threshold, approximately 3.5% of European Americans have 1% or more African ancestry (Figure S8). Individuals with African ancestry are found at much higher frequencies in states in the South than in other parts of the US: about 5% of self-reported European Americans living in South Carolina and Louisiana have at least 2% African ancestry. Lowering the threshold to at least 1% African ancestry (potentially arising from one African genealogical ancestor within the last 11 generations), 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 (Figure S8).