Who: Researchers used the Framingham Heart Study, which has compiled information on people in the Massachusetts town since 1948 and has generated more than 1,000 medical studies on risk factors for heart disease, stroke and other conditions.
Why? They used the information they collected to study the mating patterns of 879 spouses from three generations of European and Ashkenazi ancestry: those married after World War Two, their children, and their grandchildren.
What they found: The oldest generation of white, married couples often had the same ancestry and were genetically similar because they chose mates from the same background. But this similarity from "common ancestry" eroded slightly with each successive generation as spouses married outside their local community and social networks of people like them.
Why it matters: If the genetic makeup of a population is too similar, it can skew the results of tests designed to assess the degree to which a disease is passed from generation to generation through genes. With the arrival of personal genetic testing, researchers have raised concerns about "false positive" results that could lead people to take unnecessary action to address what they perceive to be a risk to their health.