DNA Reveals the Growing Ancestral Diversity of the United States

Today, over 60 million Americans have taken so-called genetic ancestry tests (offered by companies like Ancestry.com and 23andMe), which categorize the similarity of a person's genome to various reference populations from around the world. Race and ancestry, while distinct concepts, have an intertwined and fraught history in the United States. Though once firmly rooted in biological definitions, race is now widely understood to be a socially constructed category based on a range of physical and social characteristics. In contrast, ancestry describes kinship and an individual’s place in the expansive family tree of humanity. Importantly, while a person’s racial identity or classification may change across place and time, their ancestry remains fixed. In this study, we leverage nationally representative data from the Health & Retirement Study (n= 10,819) and the National Longitudinal Study of Adolescent to Adult Health (n=8,162) to explore the dynamic relationship between race and ancestry over time. We leverage genotype data and genomic methods to obtain individual-level measures similar to those provided by popular genetic ancestry tests, which we call genetic similarity proportions. We then compare these genetic similarity proportions across three distinct American birth cohorts: 1945, 1980, and 2015. We find evidence of meaningful average changes in genetic similarity proportions among members of the same self-identified racial group over time. Moreover, in some cases, the relationship between an individual’s genetic similarity proportions and their racial identification has also changed. Overall, we find support for an idea that we term ancestral diffusion: that over time, the U.S. has become increasingly diverse in terms of ancestry, even within racial groups. Taken altogether, our results illustrate the ever-evolving nature of racial conceptions and boundaries.