Judaism is matrilineal in that if your mother is Jewish, then you’re Jewish. That is, Jewish identity passes through the mother and into the child. It doesn’t matter whether your father is Jewish—at least it doesn’t matter in and of itself, for simply having a Jewish father doesn’t automatically make you Jewish. It’s only your mother’s identity that (automatically) matters.

There are four possibilities:

1. Non-Jewish father, non-Jewish mother. Not (automatically) Jewish.
2. Jewish father, Jewish mother. (Automatically) Jewish.
3. Non-Jewish father, Jewish mother. (Automatically) Jewish.
4. Jewish father, non-Jewish mother. Not (automatically) Jewish.

By analogy: White-American identity is bilineal in that if both of your parents are white, then you’re white. There are again four possibilities:

1. Non-white father, non-white mother. Not white.
2. White father, white mother. White.
3. Non-white father, white mother. Not white. (For example, Barack Obama, with his black father, is famous as the first black president of the United States of America, despite his white mother. The identities of white and black are such that white + black = black.)
4. White father, non-white mother. Not white.

Consider that for the Jews, the Jews are in-group and everybody else is out-group. Consider also that marrying endogamously is marrying in-group and marrying exogamously is marrying out-group.

Those considerations are just about the definitions of the terms “in-group,” “out-group,” “endogamy,” and “exogamy.” But with those definitions in mind, consider that the matrilineality principle among the Jews categorizes anybody with a Jewish-in-group mother as also being Jewish-in-group, which in turn makes that person Jewish-endogamy-eligible. By analogy: The bilineality principle among “the white Americans”—somehow the phrase “the Jews” looks and sounds normal but the analogous phrase “the white Americans” doesn’t—categorizes anybody with both a white father and a white mother as white-in-group, which in turn makes that person white-endogamy-eligible. The question of whether you’re Jewish or white is in part a question of endogamy-eligibility.

What are the reasons, though, that Jewish identity is matrilineal and white-American identity is bilineal? On Jewish identity, consider that mothers pass down tradition more reliably than fathers—at least that’s a plausible hypothesis or explanation. That is, matrilineality makes sense because a child with only a Jewish mother is more likely to take on Jewish tradition than a child with only a Jewish father. Consider the extremes: Somebody with no Jewish lineage except their mother’s mother (or mother’s mother’s mother) is more likely to have been given at least a modicum of Jewish tradition than somebody with no Jewish lineage except their father’s father (or father’s father’s father).

There are also other plausible reasons for why Jewish identity is matrilineal. For now, though: What about white-American identity? Why is white-American identity bilineal?