Which Relatives Are You Prohibited from Marrying?
The taboo against incest is not an outgrowth of biology. In the world of non-human mammals, it is not uncommon for a male to engage in sexual activity with its mother or sister. People who work with animals are not disgusted by such mating patterns, and the Talmud even permits Jews to mate a male animal with any female of its species, even its mother: מותר פרי עם אם, “[mating] the produce with its mother is permissible.” Human incest, however, is frowned upon today as it has been for millennia.
Ancient Near Eastern Context
Discussing the biblical incest laws, Baruch Levine, Professor (Emeritus) of Bible at NYU writes, “The prohibition of incest reflects the almost universal, natural feelings of a person towards those with whom he has been reared.” Nevertheless, not all cultures in the ancient Near East had incest laws; even among the majority that did, the exact list of who was forbidden varied. The Laws of Hammurabi and the Hittite Laws prohibit only a handful of consanguines (blood relatives) and affines (relatives by marriage).
Admittedly, a letter from the Hittite king Suppiluliuma I (ca. 1350 B.C.E.) to his brother-in-law and vassal, Huqqana of Hayasa (Armenia), states that sex with a cousin (consanguine), a (deceased) brother’s wife or a wife’s sister (affines) is sinful, even though none of these cases is mentioned in the Hittite Law Collection. Thus, law collections don’t tell the whole story.
Even so, it is striking that no ancient law collection had lists as extensive as those of Leviticus 18 and 20, detailing so many forbidden relationships. Moreover, Leviticus claims that these prohibitions differentiate Israelites from their Canaanite and Egyptian neighbors.
Not only that, the prohibited incestuous relationships in Leviticus 18 and Leviticus 20 also appear to be more extensive than the prohibitions in other biblical books, as we can see from Abraham marrying his half-sister (20:12) and Jacob marrying two sisters (19:21–30). Traditionalists explain this difference by arguing that before the revelation at Sinai, Israelites were only required to refrain from the shorter list of incestuous relations that rabbinic tradition associated with Noahide Law. Moderns, however, assume that these differences reflect changes in Israelite incest law over time.
Rabbinic Expansions of Incest Restrictions: Mostly Theoretical
The post-biblical rabbis added restrictions to the ones that were mentioned in Leviticus. For example, Moses Maimonides, following earlier Talmudic law, writes in his Mishneh Torah (Nashim, “Laws of Ishshut,” 1:6):
ויש נשים אחרות שאסורות מפי הקבלה ואיסורן מדברי סופרים והן הנקראות שניות מפני שהן שניות לעריות וכל אחת מהן נקראת שנייה, ועשרים נשים הן
There are other women who are forbidden based on tradition or on the words of the Sages, and they are called second-level relatives, since they are secondary to the main forbidden relations. Each one of them is called a “secondary” and they are twenty women [in total].
Maimonides’ list really contains more than twenty prohibitions as some of the twenty are actually references to more than one forbidden relative whom he groups together. For example, a man is forbidden to have relations with his mother’s mother, and Maimonides clarifies that this also applies to her mother and that this prohibition extends indefinitely to all of his mother’s female forebears.
Some of these rabbinic extensions fill in lacunae. For example, Leviticus 18:14 prohibits a man from having intercourse with his paternal uncle’s wife. The rabbis extend this by forbidding intercourse with his maternal uncle’s wife (#8 on Maimonides’ list). In addition, the rabbis clarify (or extend) the “uncle’s wife” prohibitions to include the wives of his parent’s half-brothers, even if they only share the same mother but not the same father (7 & 8).
Most of the extensions on this list, however, forbid unions of people who are two generations separated from one another and are not likely to occur, such as a man and his mother’s mother, as mentioned above. Some of them involve people who are three generations apart, such as a man and his wife’s great-grandmother. (This unlikely union counts as numbers 17, 18, 19 and 20 on Maimonides’ list, since everyone has four great-grandmothers.)
None of the second-level prohibitions involves a man and a woman of the same generation (e.g., first cousins) and only the two “uncle’s wife” extensions referred to above involve a man and a woman who are just one generation apart. In short, the expansions of the list by the Talmudic rabbis did not significantly restrict sexual behavior or marriage opportunities.
Karaite Expansions of Incest Restrictions
In contrast, in their early history, Karaite Jews, who do not accept the authority of Talmudic law, greatly extended the prohibition, including to unions that involve a man and a woman of the same generation. Much of the expansion came from the “catenary theory,” or in Hebrew תורת הריכוב, “torat ha-rikkuv.” As Leon Nemoy (1901–1998), a specialist on Karaism, explained:
The Karaite scholars of that period established the so-called rikkuv theory…. Exegetically and logically it was based on the assumption that man and wife form a unity of flesh (according to Gen. 2:24), from which it follows that persons related by marriage are also blood relations (sheʾer)…. In this manner, the most distant relatives came to be included in the biblical term sheʾer.
According to this theory, blood relatives of one’s spouse were considered to be one’s own blood relatives, thus turning affines into consanguines, with all the implications. One example (of many): If man A is married to woman B, her siblings would be considered the man’s siblings. As a result, the families effectively merge, and natural siblings of man A are not allowed to marry the natural siblings of woman B.
Forbidding two brothers from marrying two sisters does serve some purpose in a society that practices yibbum, levirate marriage. For example, two brothers, A and B, are married to two sisters, respectively C and D. If A dies childless, B is required by Torah law to marry C (Deut 25:5). This would mean that B would be married to two sisters, C and D, a transgression of a different Torah law (Lev 18:18). The catenary theory as practiced in the early years of Karaite Judaism did away with the problem since two brothers were not allowed to marry two sisters.
Nevertheless, a consequence of treating affines as consanguines is that the list of forbidden relations became so extensive that there was almost no one left for anyone to marry, and by the 11th century, the Karaite community was in danger of disappearing altogether. Thus, some Karaite Sages stepped in to annul the rikkuv theory, as Nemoy explains:
This extreme theory of incest was rejected by Joseph b. Abraham ha-Kohen ha-Ro’eh al-Baṣīr and his pupil Jeshua b. Judah, and was replaced by a less stringent law.
Even after this 11th century Karaite reform, however, Karaite Judaism’s list of forbidden marriages remains significantly expanded from that of the Torah. For example, in Karaite law a man may not marry his half-brother’s half-sister, even if he and she have no common parents. Ironically, rabbanite Judaism hews closer to the text of the Bible on the rules of incest than the Karaites do.
In this and some other laws, the Karaites haven’t returned Judaism to Torah law, but have developed Torah laws in surprising new directions based on an interpretive tradition that differs from that of the rabbanites. As Daniel Lasker writes,
Rabbanite and Karaite differences are mainly the result of divergent methods of interpreting the Bible. Each community has its own exegesis; each community has its own halakha.
Omissions from the Torah List of Restrictions
According to Leviticus 18:3, the Torah promotes a longer list of forbidden relationships as part of its rejection of the culture of Israel’s neighbors. Nevertheless, some relationships that are standardly considered incestuous today, and in the past, are missing from the biblical lists.
First Cousins—In many cultures, ancient and modern, marriages between first cousins are forbidden. We know that already in pre-biblical times, at least some Hittites forbade this union, as in the letter of the Hittite King, Suppiluliuma I, discussed above.
Today, more than half of the states in the United States restrict or entirely outlaw first cousin marriages. Both rabbanites and Karaites do permit cousin marriages, presumably because the Bible explicitly says that, at God’s recommendation, Zelophehad’s daughters married “the sons of their uncles” (Num 36:11)—their first cousins.
Uncle-Niece Relationships—The Torah forbids relations between a man and his aunt or even his uncle’s wife. However, the Torah does not forbid a man from having relations with his niece, the daughter of his sister or brother. Many religions do forbid such marriages, and in most English-speaking countries, they are illegal.
In rabbinic law, however, not only is the marriage of a man and his sister’s daughter permitted, the rabbis even promote it as an ideal marriage (b. Sanhedrin 76b).
תנו רבנן: האוהב את שכיניו, והמקרב את קרוביו, והנושא את בת אחותו, והמלוה סלע לעני בשעת דוחקו – עליו הכתוב אומר אז תקרא וה’ יענה.
The rabbis taught: Whoever loves his neighbors, behaves kindly to relatives, marries his sister’s daughter, and lends money to a poor person in a time of need, about such a person the Bible says (Isa 58:9), “When you call, the LORD will answer.”
Saul Lieberman, the great 20th century scholar of rabbinic Judaism, suggested that this encouragement should be understood as a polemic against sectarian Jews who forbade such unions:
The rabbis often turn a permissible action into a mitzvahas a way of countering the claims of sectarians. 
Whatever the reason, Rashbam (R. Samuel ben Meir, c. 1080–c. 1165), argued that this ideal applies also to marrying a brother’s daughter—though ironically, his brother, Rabbenu Tam (R. Jacob ben Meir, 1100–1171), argued that a marriage between a man and his brother’s daughter, אין עושה כל כך מצוה, “is not as meritorious.” 
An Ossified List
In short, with the exception of the slight expansions in the rabbinic period noted above, the list of forbidden relations in Judaism became ossified in the lists from Leviticus. As a consequence, the list in Leviticus, which in its ancient context was broader than those of most surrounding cultures, and was meant to demonstrate Israel’s “holiness” in contrast to its neighbors, has ended up being less restrictive than the prohibitions of many modern cultures.
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Prof. Rabbi Marty Lockshin is Professor Emeritus at York University and lives in Jerusalem. He received his Ph.D. in Near Eastern and Judaic Studies from Brandeis University and his rabbinic ordination in Israel while studying in Yeshivat Merkaz HaRav Kook. Among Lockshin’s publications is his four-volume translation and annotation of Rashbam’s commentary on the Torah.
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