Claude Levi-Strauss, often called the father of modern anthropology, wrote extensively on the subject of incest taboo. Yes, this profile is going to be about incest. We're squirming already.
Incest taboo, Levi-Strauss argued, is universal. All human societies prefer exogamy to endogamy, in order to strengthen previously unrelated households through newly-formed bonds of marriage. Which is all fine and good... in theory. But there seems to be one obvious exception.
Let's look at the patriarchs. Sarah is actually Abraham's half-sister. Rebecca and Isaac are second cousins. Leah/Rebecca and Jacob — first cousins. Can you feel our squirming through the computer screen?
Even today, infertility is common in Jewish couples because of all the past endogamy. And we don't have to go back to Abraham's times. Studies have determined that half of the world's eight million Ashkenazi Jews trace their origins to... four women. And two million share one ancestor, an Eve not from the Garden of Eden, but rather from 18th-century Germany or Poland.
So why is it, when every other society eschews endogamy, Jews do not? And can one make an argument that it's this inherent lack of incest taboo that made Jews into what they are — a tight-knit, family-centered group that survived all these centuries precisely because of this tightness?
We don't know. We're not anthropologists. We're just really, really uncomfortable.