In an ethnic sense, an Ashkenazi Jew is one whose ancestry can be traced to the Jews who settled in Central Europe. For roughly a thousand years, the Ashkenazim were a reproductively isolated population in Europe, despite living in many countries, with little inflow or outflow from migration, conversion, or intermarriage with other groups, including other Jews. Human geneticists have argued that genetic variations have been identified that show high frequencies among Ashkenazi Jews, but not in the general European population, be they for patrilineal markers (Y-chromosome haplotypes) and for matrilineal markers (mitotypes). However, a 2013 study of Ashkenazi mitochondrial DNA, from the University of Huddersfield in England, suggests that at least 80 percent of the Ashkenazi maternal lineages derive from the assimilation of mtDNAs indigenous to Europe, probably as a consequence of conversion. Since the middle of the 20th century, many Ashkenazi Jews have intermarried, both with members of other Jewish communities and with people of other nations and faiths.

For what period of time were the Ashkenazim a reproductively isolated population in Europe?