AT A GLANCE
Who is a Jew?
Counting Jews requires decisions about who is being counted, a direct function of who is defined as a Jew: biology, self-identity, or behavior.
Biological Jews are individuals with a Jewish parent versus those Jews who practice Judaism, with or without having a Jewish mother or father. According to Reform Judaism and the patrilineal descent of Abraham, Isaac and Jacob, a Jewish father can define a Jew. According to Orthodox and Conservative Judaism and the tradition of hundreds of years, only a Jewish mother defines a Jew. Demographic surveys usually do not ask about grandparents, although Israel's law of return includes grandparent lineage.
Self-identity is how individuals think of who they are. Some people think of themselves as Jews regardless of their parents' religion. They may have Jewish heritage or Jewish peer groups.
Behavior is what people do. Behavioral Jews practice Judaism and live as Jews, with or without biological origins. This includes people who go to synagogue weekly, observe kashrut, and otherwise meet tests of behavioral Judaism. If their parents were not Jewish or they did not formally convert to Judaism, they are usually excluded in counts.