I put this question to my “Anthropology of the Jews” class when it met for the first time on Thursday. They came up with some interesting answers.

The first, surprising to me—it’s a class that always draws a broad spectrum of Jews and others—the first definition offered was the halakhic one, the one according to Jewish law: a Jew is somebody who has a Jewish mother. I had to explain that this is not the only way to be Jewish under Jewish law—kosher conversion being the other.

But I wanted to know if they thought this was a racist, or at least racial, definition. They didn’t say so, but it’s hard to get around. Kosher conversion is a pretty big hurdle—you have to study for a long time, learn a lot, and pledge to follow Jewish practices like the kosher and Sabbath laws among many others.

Nevertheless, many people have joined the Jews over the centuries. In fact, the genetic evidence proves two things: the Jews of all nations have something in common in their genes, which links them to their Lebanese and Jordanian Arab cousins; and the Jews of all nations look at least somewhat like the people they have lived among for generations.

Which tells you that genes have flowed in, but not freely. Becoming a Jew-by-choice is a pretty serious matter and it ensures a certain commitment. Orthodox Rabbis, including those who determine the definition of a Jew in Israel, cast a pretty jaundiced eye on the many conversions triggered by marriage. As my friend the Rav (Rabbi Emanuel Feldman) likes to say, it should be about Moses, not Melanie.

Moses, however, (the Bible tells us) married a daughter of a priest of Midian, so that presumably was about both Moses and Melanie (or rather, Moses and Zipporah). Joseph married the daughter of the Egyptian priest Potiphar, whose wife had unsuccessfully tried to seduce the young man and then accused him of rape. Yet Jews have for centuries blessed their sons in the name of Joseph’s sons with Potiphar’s daughter. Esther saves the Jews of Persia by being married to the emperor, whom she hasn’t even bothered to inform that she is Jewish.

And of course, Ruth, the most famous Jew-by-choice–although she stems from the tribe of Moab, sworn permanent enemies of Israel—marries not one but (after being widowed) a second Jewish man, and through the second (a rich community leader) becomes the great-grandmother of King David and, through him, the ancestor of the Messiah.

Of course, the rabbis will tell you, it was different then. For one thing, in Biblical times Jewishness descended through the father, as it does in many religions; for another, Zipporah, Ruth, and Potiphar’s daughter were true Jews-by-choice. After a certain point the law was changed to make descent matrilineal, perhaps because many Jewish women were raped by outsiders.

In any case the Reform movement now recognizes descent through either parent, and this causes no end of difficulties in and outside of Israel. For example, a local Jewish high school here in Atlanta catered to Reform and Conservative families. The more traditional parents were worried that their kids might be dating Jews from more liberal families who were not considered Jewish according to the strict definition. In Israel, whether or not you are considered a Jew affects many aspects of your life.

Then there is the reverse-inheritance definition: A Jew is someone who has Jewish children. Once when Shimon Peres, now President of Israel, spoke in Atlanta, he said he liked that definition, but that as he got older he was thinking of amending it to: A Jew is someone who has Jewish grandchildren. The childless shouldn’t be penalized, but if you do have children, the logic is almost ironclad; what better proof than that you have passed it on?

Of course, the students also came up with definitions based on belief (for example, in one God) and practice (following Jewish law). And I reminded them of the really racial definition in the Nuremberg laws: a Jew is someone who had a Jewish grandparent. This law tragically informed countless Jews who thought they were German that leaving Jewishness behind may not be up to you.

Then there is the old wag’s definition that a Jew is anyone who says he is, because who would be crazy enough to claim it if it weren’t true?

But actually—all the romantic Melanie’s (and Michael’s) of the past notwithstanding—it has never been so easy to choose to be Jewish, because the Jews have always been under pressure, if not under siege. So it took a certain amount of character to join them. And that is part of the reason for Jewish success: Their imports were better than their exports.