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Parshat Bamidbar – Halakhic means to determine paternity

Can genetic testing override halakhic precedent and render someone a mamzer?

Iyar 5783 | May 2023

The importance of accurate paternity

Parshat Bamidbar begins with God commanding Moshe to take a census of the adult Israelite men by “their families, their patriarchal houses.”[1] A description of the Israelite camp, divided by tribe, follows the census. This census eventually determines the division of the land of Israel into proportionate tribal territories, which were then further divided by patriarchal households.

As we discussed in Parshat Emor, tribal affiliation passes through the father. The mishna teaches that when a couple in a halakhically permitted marriage has a child, the child’s religious identity and tribal affiliation is determined by the father.[2] Children born from prohibited unions are more complicated – some inherit the mother’s identity, others take on the more “damaged” status, such as mamzer or halal.[3]

Paternal identity has halakhic ramifications far beyond inheritance and tribal affiliation. Aside from the stigma and limited marriage options of a mamzer or shtuki (a child whose father is unknown), a child can be put to death for striking their father. Both the Jerusalem and Babylonian Talmud point out the difficulty posed by sentencing a person to death based on the presumption that their mother’s husband at the time of their conception is their biological father.[4] And yet, as this is the law, it’s clear that the halakhic mechanism or rationale used to determine paternity is not only deemed to be reliable but is considered conclusive.[5]

What is this halakhic mechanism?

The Babylonian Talmud teaches that paternity is determined based on the precept of “rov,” “majority.” Chazal (Talmudic sages) explain the biblical verse “side with the majority” teaches us to follow the majority in cases of doubt, specifically to issue rulings that follow the majority opinion of the rabbis in Sanhedrin or a Beit Din on matters of general halakha and specific court cases.

The Talmud in Chullin explains that the Torah rule of “rov” applies to other cases of doubt as well –if a piece of meat is found on a street where nine out of ten butcher shops are kosher, the meat is considered kosher as it is assumed to have come from the majority of kosher shops.[6] The rabbis continue to discuss the concept of “rov” – its applications and potency –bringing examples of cases where a person’s individual status is unknown but decisions are made based on the determination that it is the same as the majority of people. In this vein Rav Mari states that because “rov be’ilot akhar ha-ba’al,” “the majority of relations are with the husband,” a married woman’s husband is the biological father of her children.[7] “Rov” is reliable enough to seal the death sentence of a person who strikes their presumed father.

The Jerusalem Talmud brings the same general discussion except instead of using the halakhic precept of “rov” it uses that of “chazaka,” a presumption based on a previously known status or established practice.[8]

There’s significant rabbinic debate regarding the mechanics of rov and chazaka: What’s the difference? Is one more powerful than the other? What burden of proof is required to overrule each one? Some rabbis explain that these concepts each have multiple meanings, at times they determine reality, at others they determine a court ruling in the case of doubt – and the burden of proof is adjusted accordingly.[9] Rav Asher Weiss indicates that in this specific case the concepts of rov and chazaka are interchangeable; the statement that a married woman is mostly intimate with her husband is a behavioral norm, which means it’s both a type of chazaka and a type of rov.[10]

While there is much debate it must be noted that in this case many poskim maintain the halakhic mechanisms of rov or chazaka are more than statistical probabilities that decide which law should be applied, they determine the state of reality and create a halakhic certainty of paternity.[11] According to these opinions the burden of proof necessary to determine a child’s father is not the mother’s husband and the child is a mamzer is overwhelming.

The burden of proof

If the concept of “rov be’ilot achar ha-ba’al” is based on the assumption that the majority of an individual woman’s intimate relations are with her husband then there are times when that rule should not apply – such as the husband’s extended absence at the time of conception or the wife’s reputation as “exceedingly promiscuous.”[12]

The gemara and poskim discuss specific situations and many are reluctant to set aside the rule of “rov be’ilot” and determine that a child is a mamzer. Often there must be a combination of factors – the husband’s absence and rumors of promiscuity.

For example, if a man has been away from his wife for 11 months and there are no rumors of the wife’s infidelity the child is not considered a mamzer; the husband is considered the father of a child born within 12 months of the last time he was with its mother.[13] The rabbinic discussion of this rule clearly indicates that the rabbis were aware that a typical pregnancy lasts around 40 weeks. Some claim that 12 months is a medical possibility, some claim that there may be delayed fertilization or implantation, still others combine the possibility of a slightly longer gestation with some fancy math to stretch the weeks into months – all do so to limit the amount of children declared mamzerim.[14]

Even in more extreme cases some poskim claim the husband is still considered to be the father. One claim is that “rov be’ilot” means most relations that will lead to a pregnancy, since women who are unfaithful do so in a way that would not lead to pregnancy, so even in an individual case where a woman has a reputation as unfaithful “rov be’ilot” may still be applicable.

Others argue that the concept of “rov be’ilot” is not a specific majority of this woman’s relations, but rather a general majority of married women – since the majority of married women are mostly intimate with their husbands, most husbands are the father of the child, therefore in cases of doubt a child is considered to be part of the majority and not a mamzer.

Is blood thicker than halakha?

Blood types were discovered in the early 1900s and by the 20s, when they were understood to be hereditary, blood tests were used to determine some paternity claims; although they couldn’t determine who the father was, they could prove who the father was not. More accurate tests for determining biological relations were developed in the 1960s and 70s, but it wasn’t until DNA tests were made available in the 80s and 90s that paternity could be positively determined with almost 100% accuracy.

With each scientific breakthrough halakhic authorities were faced with the challenge of reconciling halakhic precedent with scientific knowledge and deciding whether these scientific proofs could determine halakhic status or were admissible in Beit Din.

In Tractate Nidda Chazal state that blood is inherited from the mother:

“There are three partners [making] a person: The Holy One Blessed be He, one’s father, one’s mother. The father seeds the white which the bones, sinews, nails, brain in their head, and white of the eye is made from. The mother seeds the red which is the skin. Flesh, [blood,] hairs, and black of the eye comes from.[15] The Holy one blessed be He places within breath and soul and facial expressions, the sight of the eyes, the hearing of ears, the speech of the mouth, the movement of the legs, wisdom, and intelligence.”[16]

This statement was not relegated to the field of aggadah, a non-legislative teaching; it was used as the basis of practical halakhic rulings. For example, Shulchan Aruch and Rema rule that a person who has had two boys die because of their circumcision may not circumcise any subsequent boys on the 8th day but must wait until their son is older and stronger.[17] The sages were aware that there were clotting disorders such as hemophilia that were often shared by siblings. Rema brings an opinion that this rule only applies to a woman’s sons and not a man’s sons; if a man lost two sons from one wife and remarried, a son from the next marriage should be circumcised on the eighth day.[18] Taz and Vilna Gaon explain that this opinion is based on the gemara in Nidda quoted above which states that blood is inherited from the mother.

Perhaps the first rabbi to rule on this issue, Rav Uziel refused to accept ABO blood tests to disprove paternity and render a child a mamzer. He ruled that Chazal were divinely inspired when they stated that blood exclusively comes from the mother and therefore testing blood could not determine paternity because science could not override the faithful traditions of Chazal.[19]

In the mid-1970s Tzitz Eliezer rejected the use of ABO blood tests to prove that a woman’s husband was not her child’s father.[20] Among other practical issues such as the rare possibility that a person’s blood type has changed, he states that the halakhic perspective is that blood is exclusively inherited from the mother.[21]

Several halakhic authorities reject this interpretation of the gemara, arguing that the gemara could be interpreted differently so as not to contradict scientific knowledge or that those who ignored scientific facts were “sticking their head in the sand.”[22] Furthermore, Rav Shlomo Dichovsky notes that Rambam and Tashbetz did not think that the Chazal’s scientific statements were Divinely inspired but understood that they were based in contemporary medical knowledge. Chazal are halakhic authorities, scientists are scientific authorities. Therefore, each generation should utilize the accepted scientific positions of their time as evidence in Beit Din.[23] This opinion is debated, as we saw above Tzitz Eliezer follows Rivash’s opinion that Chazal were Divinely inspired, so we should defer to their halakhic rulings even when they are contradicted by modern scientific knowledge.[24]

Interestingly, Rav Dichovsky ultimately rules that a DNA test that negates paternity creates enough of a doubt that the child loses rights to inheritance and support, but it is not a strong enough proof to negate “rov be’ilot” and declare the child a mamzer.[25]

DNA evidence

While ABO blood tests can only be used to refute paternity, DNA tests can be used to positively determine paternity with nearly 100% certainty. Can they be used to declare someone a mamzer?

Rav Ovadia Yosef ruled that one may not rely on DNA testing to establish someone as a mamzer since the rule of majority – “rov be’ilot achar ha-ba’al” – is stronger than any other statistical proof. He explains that DNA testing is also based on statistical probability and if it contradicts the statistical probability of “rov be’ilot” we continue to rely on the halakhic certainty of the latter, and the mother’s husband is considered the father.[26]

This ruling is not based on a misunderstanding of the reliability of DNA testing – there are certain factors that can lead to rare false results, chief among them human error. And although DNA testing is over 99% reliable and there are further steps that can be taken to reduce any remaining scientific doubt, it’s possible Rav Yosef purposely uses scientific aversion to declaring 100% accuracy to avoid a situation that could lead to a plethora of newly established mamzerim.

Nevertheless, one could argue that this is disingenuous. How could a 99%+ scientific probability be discounted based on halakhic presumption of majority?[27]

There are many explanations for these rulings, I will mention but two.

The first relates to the interpretation of “rov be’ilot achar ha-ba’al.” We saw that many halakhic authorities explain that this is a statement about the individual woman, while others explain that the statement applies to women in general (or at least Jewish women). Because the majority of women mostly have relations with their husbands, all children are considered to be the progeny of their mother’s husband.[28] Individual cases can’t override the generally established rule without absolute, 100% conclusive proof.

Not admissible in this court

The other possibility is based on a midrash in Yoma that discusses the manna.[29] The Torah states that the daily manna portion was an omer measurement per person, and based on the language used Chazal explain that no matter how much a household gathered – be it more or less – when they returned home they would have an omer per person. The midrash explains that this was used to determine paternity in unclear cases. For example, if a woman married another man within 3 months of her divorce and had a child 9 months later the child may be the first husband’s, born after 9 months, or may be the second husband’s, born after 7 months. If these families came before Moshe to settle paternity he told them to gather the household’s manna the next day and measure it – whoever had the extra omer was the rightful parent.

The Ben Ish Chai answers a question asked by Rav Moshe ben Chaviv – how is this “paternity test” considered authoritative? How can a Divine message similar to prophecy be used to determine halakha when neither are accepted sources of halakha – “it is not in the heavens”? He notes several possible answers, ending with the possibility that the test did not determine halakha in Moshe’s court, but after the possible biological families realized the truth they would withdraw their court claim and settle among themselves.

There is a strong, although by no means universal, rabbinic tradition that claims that even a definitive heavenly proclamation has no place in a Beit Din. The Beit Din is meant to use halakhic tools that are designed not only to determine truth, but also to lead to justice and righteousness.

The gemara in Kiddushin states that God acted charitably and righteously with Israel allowing for families of a mamzer to mix with the rest of Israel without the risk they would be identified and separated in the future.[30]  Aside from the subtle reminder that none of us can know how “kosher” our lineage is, this gemara helped to shape another strong rabbinic tradition to refrain from determining someone is a mamzer or publicizing that information.[31] While there are strict laws about whom a mamzer may marry, there is no law to reveal that someone is a mamzer. In this vein, Rav Aharon Lichtenstein related a tradition of European rabbis to be “away” for the weddings of people they knew to be a mamzer.[32]

As we have seen, some halakhic authorities refuse to accept any DNA tests. Others will accept them to identify the dead and release agunot but never to establish paternity, and still others to rule on inheritance and child support but not mamzerut. Those who refuse to determine halakhic status based on DNA evidence are not rejecting science. One might say they are choosing to ignore it in favor of halakhic precedent that has greater standing in halakhic court. If a Divine message cannot override such rabbinic considerations, neither can a scientific statement with even the most negligeable possibility of doubt.


[1] Bamidbar 1:18

[2] TB Kiddushin 66b. The language of the mishna indicates that it was composed at a time when most people had lost their specific tribal affiliation and Jewish men were identified as Kohen (Priest), Levite, or Israel (of any tribe other than Levite). The child of a Kohen man married to a Levite woman is a Kohen. The child of a man who is Israel married to the daughter of a Levite is Israel. This is a bit of an oversimplification; see the mishna and ensuing discussion in the gemara for more details.

[3] In broad strokes: A mamzer is the child born from a relationship that incurs the death penalty – such as a married woman and a man who is not her husband, or two siblings. The child of a mamzer is also generally considered a mamzer for several generations. A Kohen who marries a woman prohibited to him, such as a divorcee, loses much of his status as a Kohen – his subsequent children are halalim, and this status is passed down through the sons (although it can also affect any daughter’s  marriage prospects).

A general summary of the mishna and subsequent discussion in the gemara is as follows: In cases where the marriage is prohibited but halakhically valid the child inherits the “flawed” identity of the offending parent – a mamzer or halal. If the woman is permitted to marry another Jew but marriage to him is invalid – the child is a mazer. If the woman may not marry any Jew – such as when the woman is not Jewish – then the child’s status is determined by the mother.

[4] TB Chullin 11b; TY Kiddushin 4:10

[5] This is true in almost all cases, but rabbinic authorities discuss possible exceptions to this rule, some of which we will discuss.

[6] Shemot 23:2; Mishna Torah Hilkhot Sanhedrin v’Ha-Onashim 8:1; Sefer HaChinuch 78;

[7] TB Chullin 11b. See note 4 for exceptions.

[8] TY Kiddushin 4:10

[9] Rav Moshe Avigdor Amiel, Sefer HaMidot b’Cheker Halakha,

[10] Minchat Asher Vakikra 25; see also Trumat HaDeshen 207

[11] Responsa Rabbi Akiva Eiger Mahadura Tanyana 108

[12] Rashi and Tosafot Chullin 11b; Sota 27a, Yevamot 80b, Shulchan Aruch Even Ha’Ezer 4

[13] Shulchan Aruch Even Ha’Ezer 4:14

[14] There are medical claims of pregnancies lasting around 12 months, but they’re surrounded by doubt.

[15] While some versions of the aggadta in Nidda do not include “blood” in the list of materials the mother provides, the Gra and HaAmek She’elah both include blood.

[16] Babylonian Talmud Nidda 31a

[17] Shulchan Aruch Yoreh De’ah 263:2

[18] ibid; Rema ultimately rules that this law applies to the father as well, he does so because this is a case of doubtful mortal danger and in such cases we are lenient with the laws of circumcision (safek nefashot l’hakel) and stringent with the laws of saving a life.

[19] Sha’arei Uzziel 2:40; as to the Divine inspiration of Chazal’s medical statements see Teshuvot HaRivash 447.

[20] Tzitz Eliezer 13:104

[21] If this is the case, why do we refrain from circumcising a child whose paternal brothers died after circumcision? Shouldn’t this rule only apply to a mother’s children who share the blood? Rav Waldenberg explains that Shulchan Aruch and Rema rule apply the injunction to boys from the same father because the death may be caused by factors other than blood which may be inherited from the father.

[22] Nishmat Avraham Even Ha’Ezer 4 in the name of Rav Shlomo Zalman Aurebach who explains the gemara in Nidda could be as saying that the mother causes the production of blood and traits of the blood could still come from both parents; Rav Yitzchak Herzog, Assia 35

[23] Teshuvot Bikkurei Asher 6; Guide to the Perplexed 3:14; Tashbetz 1:165.

[24] Tzitz Eliezer does not claim modern science is wrong, but brings examples that it is always changing and not definitive. For more sources on the topic of blood tests and halakha in general see, “Establishing paternity by means of blood type testing in Jewish law and Israeli legislation,” Prof. Dov Frimer, JME Book I, pg. 240-276 Notes 44-46 there deal with sources for the debate about the scientific reliability of Chazal’s statements.

[25] “Shlilat abahut b’emtzaut bedikot HLA – psak din” Assia 35

[26] Responsa Yabia Omer, Even Ha’Ezer X

[27] When Rav Yosef wrote his teshuva tests had an accuracy of around 99.6%, today they are closer to 99.99%, but it seems unlikely this not-insignificant difference would change his ruling.

[28] Responsa Yabia Omer, Even Ha’Ezer IV 14, 15

[29] TB Yoma 75a

[30] TB Kiddushin 71a; Mishna Torah Hilkhot Melachim 12:3

[31] Rema Even HaEzer 2:5;

[32] “Blood tests and DNA in Beit Din I,” Chaim Jachter, published in Gray Matter III

Rabbanit Debbie Zimmerman

Debbie Zimmerman graduated from the first cohort of Hilkhata – Matan’s Advanced Halakhic Institute and is a Halakhic Responder. She is a multi-disciplinary Jewish educator, with over a decade of experience in adolescent and adult education. After completing a BA in Social Work, Debbie studied Tanakh in the Master’s Program for Bible in Matan and Talmud in Beit Morasha.