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Marty Lockshin





Why the Torah Prohibits Incest





APA e-journal

Marty Lockshin





Why the Torah Prohibits Incest








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Why the Torah Prohibits Incest

Although incest taboos are found in the majority of cultures, medieval Jewish thinkers found this to be an insufficient explanation for the Torah’s prohibitions. 


Why the Torah Prohibits Incest

The Prohibition of Incest

Leviticus contains two lists of sexual prohibitions, one in ch. 18 and the other in ch. 20. After an introduction in which the Israelites are warned not to behave like Egyptians or Canaanites (18:1-3), but instead to follow God’s laws (18:4-5), Leviticus 18’s set of prohibitions opens with a general prohibition:

ויקרא יח:ו אִישׁ אִישׁ אֶל כָּל שְׁאֵר בְּשָׂרוֹ לֹא תִקְרְבוּ לְגַלּוֹת עֶרְוָה אֲנִי יְ-הוָה.
Lev 18:6 None of you shall come near anyone of his own flesh to uncover nakedness: I am YHWH.

What is the reason behind the Torah’s incest laws? Leviticus 18 speaks forcefully about the problem of incest:

ויקרא יח:כד אַל תִּטַּמְּאוּ בְּכָל אֵלֶּה כִּי בְכָל אֵלֶּה נִטְמְאוּ הַגּוֹיִם אֲשֶׁר אֲנִי מְשַׁלֵּחַ מִפְּנֵיכֶם. יח:כה וַתִּטְמָא הָאָרֶץ וָאֶפְקֹד עֲו‍ֹנָהּ עָלֶיהָ וַתָּקִא הָאָרֶץ אֶת יֹשְׁבֶיהָ. יח:כו וּשְׁמַרְתֶּם אַתֶּם אֶת חֻקֹּתַי וְאֶת מִשְׁפָּטַי וְלֹא תַעֲשׂוּ מִכֹּל הַתּוֹעֵבֹת הָאֵלֶּה הָאֶזְרָח וְהַגֵּר הַגָּר בְּתוֹכְכֶם. יח:כז כִּי אֶת כָּל הַתּוֹעֵבֹת הָאֵל עָשׂוּ אַנְשֵׁי הָאָרֶץ אֲשֶׁר לִפְנֵיכֶם וַתִּטְמָא הָאָרֶץ.
Lev 18:24 Do not defile yourselves in any of those ways, for it is by such that the nations that I am casting out before you defiled themselves. 18:25 Thus the land became defiled; and I called it to account for its iniquity, and the land spewed out its inhabitants. 18:26 But you must keep My laws and My rules, and you must not do any of those abhorrent things, neither the citizen nor the stranger who resides among you;18:27 for all those abhorrent things were done by the people who were in the land before you, and the land became defiled.

And yet the chapter never really makes the idea behind these laws clear, only that violating them is terrible. As Jacob Milgrom notes:

Motives are attached to all the laws [in chapter 18]. These, however, are words of derogation and disgust. In addition, the laws are rationalized in several ways: they stem from God (v. 4); they lead to life (v. 5); the land will not tolerate their violation (vv. 25, 27, 28); and their violators will be defiled (v. 24) and cut off (v. 29)…. These reasons, however, do not penetrate to the quintessential rationale that lies behind the laws.[1]

We could add to Milgrom’s list of the Bible’s rationalizations its claim that the laws of incest are yet another way of differentiating Israelites from the Canaanites and Egyptians, though the accuracy of this claim is hard to gauge. We do have ANE law codes from the Babylonians and the Hittites that forbid various forms of incest,[2] but we know little about Canaanite sexual mores.[3] Moreover, although Egyptian culture strongly condemned adultery and rape, it is unclear if they had incest taboos.[4]

Even if the claim about differentiating Israel form Canaanite and Egyptian practices reflects some historical reality, it does not explain why Leviticus considered it so serious that it defiled the land, leading to the Canaanites being expelled, and that such behavior on Israel’s part will have the same consequences. Even though people feel an intuitive loathing to incest, many traditional commentators grappled with the attempt to find rational or philosophical explanations.

A Divine Fiat (Ḥok)

In his introduction to his commentary on Mishnah Avot called “Eight Chapters,” Moses Maimonides (1138-1204) suggests that the incest laws are an example of laws in the Torah whose reasons are considered “unknown.” (This does not stop him from offering possible reasons in his Guide of the Perplexed, as noted below.[5]) In other words, this is one of those apparently inexplicable Israelite practices, such as forbidding the consumption of certain animals or forbidding certain types of clothing.

The suggestion appears in a passage in which Maimonides wishes to solve what looks like contradictory stances taken by the Sages and the philosophers regarding whether it is better for a person to be someone who desires sin but controls his urges (the Sages’ position, [Sifra, Kedoshim 4.9.12]) or whether a truly great person has no desire to sin at all (the philosophers’ position).

Maimonides argues that this is not really a debate, but that each group is thinking about a different kind of law. The philosophers are speaking about “rational prohibitions” (משפטים), which people should intuit to be wrong and should not desire to do at all, whereas the Rabbis are speaking about “divine fiat” prohibitions (חקים), with which people must comply without knowing why, simply because God commands it:

The instances they [= the Sages] cite are all from the ceremonial law, such as partaking of meat and milk together, wearing clothes made of wool and linen, and incest. These, and similar enactments are what God called (Lev 18:4) “my statutes” (חקותי)… (Gorfinkle trans. with adjustments).

The laws of incest, however, cannot easily be understood as serving the same function as, for example, the laws of kosher food, i.e., laws that are peculiar to Israel, since such laws are broadly cross-cultural; as Baruch Levine writes, “The prohibition of incest reflects the almost universal, natural feelings of a person towards those with whom he has been reared.”[6]

A Kabbalistic Secret (Nahmanides)

A related approach is to claim that the reason for prohibiting incest is a secret. For instance, Moses Nahmanides (1194-1270) acknowledges that the Bible never explained the rationale for the rules of incest. This inspires him to suggest that the explanation is likely a kabbalistic secret of Jewish mysticism:

ואין בידנו דבר מקובל בזה, אבל כפי הסברא יש בענין סוד מסודות היצירה
We have no tradition as to [the reason for] this prohibition of forbidden relationships, but logically it would appear that this matter is one of the secrets of creation.[7]

He concludes that only God truly knows the reason for these laws, and God shares the reasons only with חכמי יועציו—his wise counsellors, presumably a reference to those initiated into the world of Jewish mysticism. (This is only one of Nahmanides explanations; see below.)

Scientific Explanation

With our current understanding of genetics, the biological problem with inbreeding is well-known. Some contemporary scholars suggest that this is the origin of incest taboos, namely, that through the processes of evolution and the survival of the fittest, humans who had an aversion to incest—and who were accordingly the most likely to produce healthy progeny—were the ones most likely to survive.[8]

Some version of this idea seems to have existed in the medieval period, since Abraham ibn Ezra (1089-1167) attacks it. On the verse that describes the marriage of Moses’ parents (Exod 2:1), he says:

…והיא אחות אביו. ולא דבר נכונה האומר כי תבואות השדה הנזרעת בשדה אחרת תצליח, רק באותו שדה עצמו לא תצליח כראוי[9]
…and she [Yocheved, the woman whom Amram married] was his father’s sister [a union forbidden in Lev 18:12]. Whoever claims that the produce grown in one field succeeds when planted in another field, but does not succeed [when planted] in the same field where it grew, spoke incorrectly.

The theory ibn Ezra is attacking is based on an analogy with crop rotation. The preferred procedure for successful crops is to plant seeds in a different field from the one in which they were grown. When he says that the person who made such a claim “spoke incorrectly,” he means that applying the accepted principles about plants to an understanding of the Torah’s laws of incest is incorrect.

You should not think, says ibn Ezra, that the Torah outlawed unions of relatives because the resulting children would be defective. Ibn Ezra considers Moses, Aaron, and Miriam the best proof for his argument—surely they were not defective!—and so he includes this comment specifically at this point in his commentary.

This criticism did not stop the later scholar, R. Moses Nahmanides (1194-1270)—cited above in connection with a different explanation—from advocating for the biological concern that children of incestuous unions could be defective as a strong possibility:

וכן אשר איננו טוב בקיום ולא יצלח בו תאסור אותו התורה
Where [the union is such that] the child therefrom will not have a healthy existence, nor succeed from it, the Torah prohibited such a union.[10]

Incest Is Too Attractive

Another approach familiar in modern times is the psychoanalytical theory that claims that we all have very strong if unacknowledged sexual attraction to our close relatives and that strict incest taboos were created to overcome these feelings.[11] Although psychoanalysis was only developed in the twentieth century, the idea that people might really be (too) attracted to relatives was intuited by some medieval scholars, and used to explain the incest taboos in a way similar to the Freudian explanation.

Bekhor Shor

In his gloss on Lev 18:6, R. Joseph Bekhor Shor (12th cent.) suggests that allowing incest would make everyone mired in sexual activity from childhood.

[איש איש] אל כל שאר בשרו לא תקרבו לגלות ערוה – שלא יהו ישראל מצויים אצל זימה.
“[None of you] shall come near anyone of his own flesh to uncover nakedness” – so that the Israelites not be found always mired in promiscuity.
שאם היה אדם מותר בקרובותיו, מתוך שהם מצויים זה אצל זה, ירבו זימה, לא תצא אשה בתולה מבית אביה שלא יבאו עליה אביה או אחיה או זקינה המגדלה, והבן שיהא מצוי אצל אמו ואצל דודתו.
For if a person were permitted to lie with his relatives, he would be having sexual relations with them all the time since they are so close. No woman would ever leave her father’s home a virgin, as her father, her brother, and her grandfather who brings her up, would lie with her. And a son would be lying with his mother and his aunt.

Moreover, Bekhor Shor argues that allowing incest could endanger any possibility of a secure marriage in the future,

ואפילו כשתהיינה אשת איש, יהא רגיל לבא עליה, ומלאה הארץ זימה וטנופת.
And even once she is a married woman, [the male relatives] will come and lie with her, and the land will become filled with promiscuity and foulness.

No marital bond can ever really be firm if a woman inevitably entered marriage with a sexual history with her own relatives, with whom her bonds are older and deeper than those she has with her husband.


At approximately the same time, Moses Maimonides offered a similar suggestion[12]:

All illicit unions[13] with females have one thing in common: namely that in the majority of cases these females are constantly in the company of the male in his house and that they are easy of access for him and can be easily controlled by him.[14]
…Consequently, if the status of the women with whom union is illicit were that of any unmarried woman, I mean to say that if it were possible to marry them and that the prohibition with regards to them were only due to their not being the man’s wives, most people would have constantly succumbed and fornicated with them. However, as it is absolutely forbidden to have intercourse with them, the strongest deterrents making us avoid this, …men are safe from seeking to approach them and their thoughts are turned away from them.
It is very clear that relations are easy with all women included in the prohibitions concerning illicit unions. For it is very general that if a man has a wife, her mother, her grandmother, her daughter, her granddaughter, and her sister will be in his house most of the time, so that the husband will constantly meet them whenever he enters, goes out, and is engaged upon his business.
A wife also is often in contact with her husband’s brother, his father, and his son. It is likewise manifest that in most cases a man is often in the company of his sisters, maternal and paternal aunts, and the wife of his paternal uncle, and is brought up together with them. Now these are the women with whom union is illicit because of their being relatives.[15]

Cut Down on Sexual Activity

Another explanation, specifically popular among the philosophically-minded medieval scholars, is that the laws were meant to cut down on sexual activity, since it is a base act.

Abraham ibn Ezra

At the end of his comment on the marriage of Amram and Yocheved cited above, ibn Ezra writes:

כי עיקר איסור ערוה להיות ישראל קדושים:
The principle behind the laws concerning incest is that Israelites should be holy.

This cryptic comment may be unpacked when we look at his commentary on Lev 18, which explains being “holy” as meaning refraining from frequent sexual activity:

בעבור שיצר לב האדם כבהמות, לא יתכן לאסור כל הנקבות, והנה אסר כל הנמצאות עמו בכל עת
Since humans have the same base impulses as animals, outlawing all [sexual conduct of men with] women is not possible. So the Torah forbade sexual conduct with those women who are regularly in a man’s vicinity.[16]

In other words, in order to cut down as much as possible on sexual activity, the Torah forbade sexual conduct between a man and all the women most easily accessible to him.[17]


A few years after ibn Ezra, Moses Maimonides offered a similar explanation,[18] explicitly invoking Aristotelean philosophy and ethics, which had a negative attitude to sexuality (Guide, 3:49).

The philosophers consider this matter in the same way. I have already made known to you the text of Aristotle’s saying that this sense is a disgrace to us, he means the sense of touch, which leads us to give preference to eating and sexual intercourse. In his writings he designates the people who give preference to sexual intercourse… as abject, and expresses at length his blame of them and his mockery of them.

Moses Nahmanides criticizes this position:

וזה טעם חלוש מאד, שיחייב הכתוב כרת על אלה בעבור המצאן עמו לפעמים, ומתיר שישא אדם נשים רבות למאות ולאלפים. . . . ואין לאדם נשואים הגונים כמו שישיא את בתו לבנו הגדול ממנה וינחילם בנחלתו ויפרו וירבו בביתו
But this is a very weak reason, that Scripture should make a person liable to the punishment of excision [karet] in the case of these forbidden relations, just because these women are sometimes found together with him, and at the same time permit a man to marry many women, even in the hundreds and thousands! …[If there were nothing intrinsically wrong with such a union,] a person could not do better than to give his daughter in marriage to his elder son, and they would inherit his possessions and multiply and increase in his house.[20]

Nevertheless, this view remained popular among the philosophically inclined. For instance, Rabbi Joseph ibn Kaspi (1280-1345), a philosopher who followed Maimonides’ lead on many issues, here too accepted Maimonides’ approach that the laws of incest were meant to cut down on sexual activity, and added:

אילו נמעיט עוד יהיה יותר מועיל
If we were to restrict sexual activity even more [than the Torah did] it would be better.

Sociopolitical Explanations

A sociopolitical approach emerged in the fourteenth century, advocated by two Jewish thinkers from the south of France.

ibn Kaspi

Rabbi Joseph ibn Kaspi said that observing the rules of incest serves a social function:

…כדי לשום עצת שלום בינינו ותסור הקטטה ממנו וינוח השלם… ולכן אסרה התורה הנקבה שתפול בעבורה הקטטה ולכן אסרה אישה ובתה ושתי אחיות כי אלו יקנאו זו בזו כי הקנאה יותר עצומה בקרובים ובקרובות מזולתם
[These laws are] in order to increase serenity and to remove fighting from among us so that the righteous man finds tranquility… Accordingly the Torah forbade [marriage with] a woman that might lead to arguments. [For example,] the Torah forbade [marriage with] a woman and her daughter and [marriage with] two sisters. These women are likely to be jealous of each other since jealousy is more intense among relatives, male or female, than among strangers.[21]

Nissim of Marseilles

A more uplifting explanation of the rationale for the laws of incest is provided by ibn Kaspi’s contemporary, Rabbi Nissim of Marseilles (14th century):[22]

ואפשר היות הטעם באיסור הדבק בעריות—בעבור שיתחברו אנשי המדינה הזרים אלו עם אלו, ויתחדש בסבתם קשר אהבה וקורבה עד שתהיה המדינה כולה כאיש אחד. ולא תחלק למשפחות חלוקות נבדלות אלו מאלו ותתחדש מריבה וקטטה ביניהם. ואמר אפלטון כי המדינה החשובה היא אשר ידמו אנשיה אחים ...
Possibly the reason that incestuous relations were prohibited is so that people in the nation who are not related become closer to each other. [When people marry someone from outside of their immediate family] bonds of love and kinship are forged between them; as a result, the nation becomes united like one family [literally like one person]. This way, they do not have separate families that keep away from each other leading to more fights and feuds. As Plato said, in the ideal nation, people consider each other brothers ...[23]

Marrying within one’s own family, Nissim argues, perpetuates a society where clans feud. When clans marry outside of their family, society as a whole benefits.

Nissim then expands his argument from the social to the political:

מן הראוי להם לבחור מנהיג זהב בן זהב, הוא חשוב בן המנהיג החשוב. ואם יהיה בן המנהיג נחשתי, שירחקוהו ויבחרו להם מנהיג זהב בן נחושת, כי כולם הם אחים, אם אחת ואב אחד לכולם. לכן מהכרח יישוב המדינה להזדווג אלו באלו עד שתקשר המדינה ולא תחלק למחלוקות ומריבות, "ותקיא הארץ את יושביה."
Ideally people should choose a leader who is “gold, the son of gold.” In other words, a worthy person who is the son of the previous worthy leader. But if the son of the leader is not gold but copper, they should reject him and choose as their leader someone who is gold, the son of copper. All people are brothers in any case; all [ultimately] have the same mother and the same father. That is why [people from different families] should marry each other for the necessary good of the nation. Then the nation is united and not divided into warring factions, such that “the land spews out its inhabitants” (Lev 18:25)

Samuel David Luzzatto

In the nineteenth century, Samuel David Luzzatto (1800-1865), who did not know Nissim’s Torah commentary, came to a similar conclusion,[24]at least about one of the incestuous relations outlawed in Leviticus 18 (gloss on v. 6):

איסור האחות נראה שהוא להצלחת המדינה, שאם היתה האחות מותרת היו רוב בני אדם נושאים אחותם, והיתה כל משפחה כעם בפני עצמו, ולא היו המשפחות מתחתנות ומתערבות זו בזו, ולא היתה האומה לעם אחד אלא לעמים רבים בלתי קרובים זה לזה ובלתי אוהבים זה את זה.
The prohibition against [marrying one’s] sister appears to be for the benefit of the nation. If it were permitted for a man to marry his sister, most men would. Each family would then become like a separate people. Families would neither intermarry nor mingle with each other. The nation would not consist of one people but many different peoples that had no contact with each other and did not like each other.

As we see, Jewish thinkers over the centuries struggled to find a rationale for incest taboos. But Nissim and Luzzatto creatively managed to come up with an inspiring explanation: these laws help build a better country where people see all clans as related. And a society like this is one that will last. The land, as Leviticus 18 says, will not spew out the inhabitants who build such a society.


September 27, 2017


Last Updated

October 9, 2019


View Footnotes

Professor Rabbi Marty Lockshin is a professor at York University and is currently the Chair of the Department of Humanities. 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 five volume translation and annotation of Rashbam’s commentary on the Torah.