Barzilai centered his studies on Ashkenazim not because they live longer or produce more centenarians than other ethnic groups. They don’t. It’s that their unusual development as a homogeneous community makes them easier to study at the level of DNA. Genetic research done by Barzilai’s Einstein colleague Gil Atzmon suggests that Ashkenazim branched off from other Jews around the time of the destruction of the First Temple, 2,500 years ago. They flourished during the Roman Empire but then went through a “severe bottleneck” as they dispersed, reducing a population of several million to just 400 families who left Northern Italy around the year 1000 for Central and eventually Eastern Europe. Though their numbers increased dramatically once there, to some 18 million before the Holocaust, studies suggest that 40 percent of today’s Ashkenazim descend from just four Jewish mothers. How proud those mothers would be to know that the reason their mishpocheh has remained far more genetically alike than a random population—Barzilai says by a factor of at least 30—is that until recently their sons almost never married outside the clan. http://nymag.com/news/features/ashkenazi-jews-2011-11/index2.html