Sex bias in ancestry contributions, often assessed through ancestry of mtDNA and Y chromosome haplogroups, is also manifested in unequal estimates of ancestry proportions on the X chromosome, which has an inheritance pattern that differs between males and females. The X chromosome more closely follows female ancestry contributions because males contribute half as many X chromosomes. Comparing ancestry on the X chromosome to the autosomal ancestry allows us to infer whether that ancestry historically entered via males (lower X ancestry) or by females (higher X ancestry). Under equal ancestral contributions from both males and females, the X chromosome should show the same levels of admixture as the genome-wide estimates. To look for evidence of unequal male and female ancestry contributions in our cohorts, we examined ancestry on the X chromosome (NRY region), which follows a different pattern of inheritance from the autosomes. In particular, estimates of ancestry on the X chromosome have been shown to have higher African ancestry in African Americans.9 We calculate ancestry on the X chromosome as the estimate of ancestry on just windows on the X, and we compare to genome-wide estimates (which do themselves include the X chromosome). It should be noted that these calculations differ among males and females, because the X chromosome is diploid in females and thus has twice as many windows in calculation of genome-wide mean proportions. However, our results still allow a peek into sex bias because the overall contribution of the X chromosome to the genome-wide estimates is small. We note that because our ancestry estimation method conservatively assigns Native American ancestry, we expect that much of the remaining unassigned ancestry might be due to Native American ancestry assigned as broadly East Asian/Native American, which is not included in these values (see Figure 5 in Durand et al.33).