as forbidding even marriage to a wife's former husband's wife. At least all Talmudic opinions agree that it was theoretically permitted to marry a stepfather's wife (excepting the mother), and to marry a stepsibling (a man marrying his stepsister, etc.). Nevertheless, some of the opinions expressed in the Jerusalem Talmud argue that to avoid observers jumping to the wrong conclusions, marriage between stepsiblings, or between a man and his wife's stepmother, should be forbidden, or at least carried out somewhere that observers would not already know that the participants are step-relations. What is clear, is that no opinion in the Talmud forbids marriage to a cousin or a sister's daughter (a class of niece), and it even commends marriage to the latter – the closer relation of the two. The implied support for marriage between cousins appears to have historically been taken to heart; in 19th century England, the proportion of Jewish marriages occurring between cousins was 3.5 times higher than for the marriages of other religions; in the 19th century Lorraine the proportion was twice as high as that for Roman Catholics, and 12 times higher than that for Protestants. In practice Marriages forbidden in the Bible were regarded by the rabbis of the Middle Ages, the Rishonim as invalid – as if they had never occurred; any children born to such a couple were regarded as mamzerrim "bastards", and the relatives of the spouse were not regarded as forbidden relations for a further marriage. On the other hand, those relationships which were prohibited due to qualifying as seconds, and so forth, were regarded as wicked, but still valid; while they might have pressured such a couple to divorce, any children of the union were still seen as legitimate. In general, the Jerusalem Talmud is more restrictive in regard to incest than the Babylonian Talmud; Ashkenazi Jews, following Joseph Karo, generally follow the incest regulations of the Jerusalem Talmud, while Sephardi Jews, exemplified by Maimonides, tend to follow the Babylonian Talmud. Thus Jacob ben Meir deliberately wrecked a wedding, stopping the marriage and spoiling the banquet and celebrations, because the man would have married his father-in-law's wife. The classical rabbis regarded the incest regulations as being too important and too open to misinterpretation to be taught in public, instead requiring that, when it is taught, it must be taught to each student individually. They also argued that on occasions