Ashkenazi Jewry has not practiced polygamy since Rabbenu Gershom's ban in the 11th century "except in exceptional circumstances requiring obtaining in advance the permission of 100 rabbis." Some Sephardi and Mizrahi groups, in particular those from Yemen and Iran (where polygamy is the cultural norm), only discontinued polygamy much more recently, for non-religious reasons. When these groups immigrated to the State of Israel after its 1948 creation, existing polygamous families were "grandfathered" in. Polygamous marriage is banned in the State of Israel, however, and no new polygamous marriages are permitted among those groups.