Several lines of evidence suggest that Native American and African segments represent true signals of Native American and African introgression that occurred after the transcontinental migrations beginning in the 1500s. Validation of our self-reported survey data across two independent surveys shows that self-reported ancestry consistency is remarkably high. African ancestry in European Americans is not likely to be driven by survey errors because the number of European Americans with African ancestry is ten times larger than our estimates of survey error rates. Furthermore, the ancestry profiles of self-reported European Americans with African ancestry are distinct from all other cohorts: their African ancestry is much lower than for a random sample of African Americans, and the majority of these individuals do not carry any appreciable amount of Native American ancestry, distinguishing their ancestry profiles from Latinos (see Figure S1C).