Selection of Populations
The ancestries of 23andMe customers, and therefore the demographics of the database used for this study, largely reflect the demographics of the US, as tallied in the 2010 US census. Our study considers three cohorts that comprise the three largest self-identified groups in the US, which are likewise well represented in the 23andMe database. In this study, we focus on the distribution of European, African, and Native American ancestries and European subpopulation ancestries. These populations were selected because we had available reference data sets, allowing for accurate estimation of ancestry proportions, they reflect the major waves of migration into the US just after the era of transcontinental travel began, and they are found at mean frequencies of more than 1% in our cohorts. At present, we are unable to delve deeper into the complexity of, and subancestries within, Native American and West African populations. Our resolution reflects the current availability of reference data sets from different regions.
However, we emphasize that these groups and ancestries are only a fraction of the diversity found within individuals living in the US, and as data set sizes grow, future work should extend to include analyses of other worldwide ancestries and populations and their distributions across the US.