We find evidence that sex-biased admixture processes are widespread in US history in European Americans as well as in African American and Latino populations. Estimates of proportions of males and females from each ancestral population (Table S4) suggest that under a simple demographic model of admixture, European Americans might have ten times as many female Native American ancestors as male, and African Americans might have four times as many female Native American ancestors as male. Sex bias in ancestry contributions might have been driven by unbalanced sex ratios in immigration frontier settings,76 exploitation,77 or other social factors.