Is Being Jewish an Ethnic or Religious Demographic Characteristic?

Among social scientists, Judaism is known as an ethnic religion. This means that to be Jewish you have to be born into it or decide on your own that you want to convert to it from some other religion. This is distinct from proselityzing religions, such as Christianity and Islam, that actively recruit new members. Of course, Christianity and Islam both build on Judaism, so some of the basic tenets of the two most populous religions in the world (Christianity and Islam) are derived from Judaism. The other major ethnic religion in the world is Hinduism, from which Buddhism sprang. I mention these things as preface to a story in this week's Economist about "Who is a Jew?"

Who is a Jew? This question is becoming ever more pressing for Jews around the world. It looks like a religious issue, but is bound up with history, Israeli politics and the rhythms of the diaspora. Addressing it means deciding whether assimilation is a mortal threat, as many Jews think, or a phenomenon to be accommodated. The struggle over the answer will shape Israel’s society, its relations with Jews elsewhere, and the size and complexion of the global Jewish community.

The Economist story draws from a recent survey from Pew Research, whose report has the same title of "Who is a Jew."

“Who is a Jew?” This is an ancient question with no single, timeless answer. On the one hand, being Jewish is a matter of religion – the traditional, matrilineal definition of Jewish identity is founded on halakha (Jewish religious law). On the other hand, being Jewish also may be a matter of ancestry, ethnicity and cultural background. Jews (and non-Jews) may disagree on where to draw the line. Is an adult who has Jewish parents but who considers herself an atheist nevertheless Jewish, by virtue of her lineage? What about someone who has Jewish parents and has converted to Christianity? Or someone who has no known Jewish ancestry but is married to a Jew and has come to think of himself as Jewish, though he has not formally converted to Judaism?

Of particular interest in the Pew report is that Orthodox Jews in the US are likely to be politically Republican, while other Jews are predominantly Democrats. I've noted before the demographic differences in New York and Israel, as well, between the Orthodox population and other Jews. 

My favorite "takeaway" from the Economist article, however, is the graph below, prepared by Sergio BellaPergola, who is Israel's premier demographer, showing the distribution of the Jewish population (presumably by self-definition--e.g., "ethnicity") in the world. If you think about the fact that there are more than a billion Christians and more than a billion Muslims, the number of Jews is really quite remarkable.