Economist Evelyn L. Lehrer states in The New York Times:
During the first five years of marriage, the divorce rate for a couple of the same religion hovers around 24 percent, no matter what that religion is. But it jumps to 38 percent for a marriage between a mainline Protestant and a Catholic and 42 percent for one between a Jew and a Christian
You can read a more detailed paper at Lehrer’s website. Nothing too weird, and we shouldn’t read much into it as there are all sorts of variables that influence the type of person who marries outside their own religion/ethnic group…but, I am struck that most of the marital dissonance between Protestants and Jews also seems to crop up between Catholics and Jew. Genetically it seems clear that Roman Catholicism and Protestantism form a clade with multiple shared derived characters when compared to the outgroup of Judaism, but sociologically in the Catholic-Protestant-Jew trichotomy it is the Protestants that have been the odd ones out in relation to the two white “ethnic”religious groupings. In fact the Jewish and Catholic elites were both united against the WASP establishment until 1950. The diminution of the more crass forms of nativism and a more abstract and intellectual critique of Roman Catholicism non-religious Jews could agree with, and, the subsequent rise of “cultural issues” shattered the Catholic-Jewish alliance which crested during the second heyday of the Klu Klux Klan.
But the bigger point might be that the nature of difference is less important than the difference in the first place. Group identity and support which dissolves with an outmarriage might be more important than the multiple stresses introduced by pairwise differences between individuals, as it seems that least ideologically Catholics and Protestants should share more because of their common Christianity.