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Why isn't DNA seen as a valid source of Halachic decision making? Is it because it's too small for the naked eye? A margin of error problem? What?

Normally, physical evidence is plenty of information to make halachic decisions off of. For example, a piece of unmarked meat found in a Jewish neighborhood is assumed kosher, because it's in a Jewish neighborhood. And the gemorah responds, to the person who says he wished he had a drawing tablet to draw the tzizit of the generation of the desert, by saying that if he had just memorized it in his head, that would have been good enough!

Source for the question: http://www.jewishpress.com/pageroute.do/49607

Some of the authors in Contending with Catastrophe recount that several rabbis deliberated at length concerning the reliability of DNA evidence in identifying dead bodies or body parts (with a few even coming to the conclusion that it cannot be relied upon exclusively). Isn't the reliability of DNA evidence a well-known fact that was settled by scientists and statisticians a long time ago? How is it that rabbis in 2011 are spending their time on this matter? Don't their actions give ammunition to those Jews who argue that rabbinic leaders are behind the times?

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What is the source that DNA is invalid as a source of information in halacha? – WAF Sep 12 '11 at 10:48
found a source for you. – avi Sep 12 '11 at 11:16
A reporter's question in an interview is rather a weak source. Incidentally, I wonder whether the DNA in a piece of beef can be relied upon to show it came from a steer rather than another animal and is kosher (assuming you somehow know that any beef around is kosher). – msh210 Sep 12 '11 at 15:37
If your answer is that DNA is a valid source of halachic decision making than answer as such. I've seen this said all over the place, I appologize that my random source wasn't good enough for you. – avi Sep 12 '11 at 19:41
I don't know (except from answers below) that it is valid. I also note that you didn't say, in the question, that's not valid. (You only said it's not "seen as" valid.) Nonetheless, in reply to a request (in a comment above) for a source that "DNA is invalid" you added the Jewish Press link. I think I'm justified in saying that that's a weak source for saying DNA's invalid. (Again, though, your question never says it's invalid.) I beg your pardon if I sounded snide, above; I certainly didn't feel snide, and didn't mean to sound it. – msh210 Sep 12 '11 at 21:05
up vote 7 down vote accepted

I haven't studies the sources inside, but a couple observations, and some places to do more research:

  • There are several different ways that DNA may be used in Halacha, and these ways may have different laws.

    • There's using DNA for paternity tests in cases of inheritance, which may (or may not) be different than paternity cases that concern Mamzerut. In other words, the Halacha of DNA may (or may not) be different depending on whether or not it is a judgment of money or a judgment of lineage.
    • There's also using DNA to prove that someone did or didn't murder someone. This would be used to either contradict existing witnesses or instead of witnesses. [which may, (or may not), have different Halachot].
    • There may also be other uses that I'm missing.
  • With regards to the meat. In Halacha there is a difference whether the Meat is found on the street (where we say it is permitted, since we go after the majority) and whether the meat was bought in one of the stores and the person forgot which store (where we say it is forbidden, since there is an equal probability that the person went into each store - either he did or he didn't). In halacha, these two possibilites are referred to as kol d'parish me-ruba parish and "kol kavua k'mechtsah al mechtsah dami" - see this article for a thorough breakdown [I haven't read the whole thing]. - It may very well be that DNA is considered "kavua", and if so, even the 99.9% certainty of DNA would still be considered 50/50 halachically (and again, there may be halachic differences if money or lineage is being determined.)

  • I glanced at the Talmud in Ketubot 15A, and the Gemara seems to say pretty strongly that normal leniences of majorities don't apply when it comes to lineage (Yuchsin), although I didn't really study it in depth and that may not be the conclusion.

Here are some places online that point to sources that discuss DNA in Halacha, (I haven't looked at them in depth). Follow the sources and see what they say about DNA:

  • Blood Tests and DNA - by Rabbi Chaim Jachter (links from archive.org): part 1, 2, 3, 4. - These articles were also printed in Gray Matter vol 3, and can be read on Google Books. In this series of articles, Rabbi Jachter mainly focuses on the use of DNA in paternity testing, in particular if it can be compared to blood testing for paternity testing, which is discusses in Halachic sources. It also discusses the use of DNA testing to permit Agunas after the World Trade Center Attacks, and finishes off with the discussion of whether even the Rabbis who permit DNA testing for paternity would permit using it to prove Mamzerut.
  • Shas Daf: DNA in Halacha - brings different halachic opinions about the acceptance of DNA in halacha
  • R' S.Y. Zevin in L'Ohr HaHalacha (discussed in the Shas Daf article)
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So, are you saying that DNA itself is neither valid nor invalid, but the majority of cases where it would be useful, it's not valid, because of the details of those situations? – avi Sep 12 '11 at 19:45
I guess I'm saying that DNA is used as proof. What the DNA is trying to prove may determine whether Halacha listens to it or not. In the Shas Daf article I linked to, he quotes different opinions about whether or not one may use DNA to determine paternity. However, I haven't read the sources inside so I don't know if they are trying to determine paternity for inheritance, or for Mamzerut. The Rabbis may hold that there is a difference (or they may not). For example, if a non-jew finds that he has the DNA strand that is unique to Kohanim, would we say that he doesn't have to convert? ... – Menachem Sep 12 '11 at 20:06
...Can he now do birchat Kohanim? I don't think anyone would say that. There is an idea in halacha that witnesses create the reality they are testifying about (not that they make it up, but it only becomes halachically real when they testify), which is why we listen to them. I can't remember a primary source for this right now, but it's mentioned here: chabad.org/library/article_cdo/aid/960666/jewish/… . --- Would we ever say that DNA would do the same thing? – Menachem Sep 12 '11 at 20:06
So is DNA proof, or is it only 50/50 proof? – avi Sep 12 '11 at 20:11
link to an article about the cohen gene: aish.com/ci/sam/48936742.html – Menachem Sep 12 '11 at 20:12

I think R' Broyde answered it pretty well. Technology progresses quickly, it takes time for everyone to be willing to fully accept the latest results. Not everything always ends up being certain as once thought, so sometimes some rabbis are a bit extra conservative and cautious. Also, many rabbonim are willing to rely on DNA evidence alone, so it is "seen as a valid source of Halachic decision making".

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While obviously true, it doesn't really answer my question. – avi Sep 12 '11 at 19:44
@avi, it seems to IMO. You asked why DNA isn't seen as valid, and that's one possible reason it's not seen as valid: because it's too new. – msh210 Sep 12 '11 at 21:07
Aiyn Kal Chadash Tachat Hashemesh :) Even if 'it's too new' a reason must still be given for what is new about it. Or why this new thing isn't valid. – avi Sep 12 '11 at 21:12
@avi, are you interested in getting answers to the question "Why isn't DNA seen as a valid source of Halachic decision making?" or in debating with the posekim who don't see it that way? For the former, this is a valid answer. For the latter, I suggest that you contact them directly. – Isaac Moses Sep 13 '11 at 14:17
Nobody actually says that "it's too new" is the reason. R' Broyde is saying that other people must think it's too new. Not that he himself think's it's too new. It's a guess on his part, and the people themselves have reasons. "It's too new" is a political answer, not a halachic one. – avi Sep 13 '11 at 18:22

In terms of the case of the quote of your question, when it comes to identifying a dead body for the purpose of permitting the wife/widow to remarry, there was an argument put forth to not use DNA for such. It went as follows:

The strength of DNA results is in the matching up of several loci. A match between two DNA samples in one location is not very significant. Therefore, it is only the combination of several pieces of information, each of which on their own are not overwhelmingly significant, that creates the incredibly low chance of a false-positive. (Disproving a match, on the other hand, requires much less.)

That being the case, a possible problem arises in using this information for aguna cases. The halacha in testimony for an aguna is that while a siman muvhak, a single definitive marker, is enough, non-definitive markers do not combine, no matter how many there are (Rema Even Haezer 17:24). There is a dispute if this only excludes low-quality markers (Beis Shmuel #73), or even average markers (Chelkas Mechokek cited in Beis Shmuel). However, my (brief) research seemed to indicate the odds of any one location match to be about 8%, which is not at all a significant statistic (about 1 in 12!). This being the case, the DNA evidence, which relies on the combination of non-definitive markers, would be invalid.

I would note that many prominent authorities have explicitly said that DNA evidence is valid. R' Elyashiv, cited by Nishman Avraham E.H. 4:14 (along with R' S.Z. Aurbach), R' Z.N. Goldberg in several letters, and I was present when R' Moshe Heinemann said as much.

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DNA is valid evidence for what? e.g. ritual matters, monetary matters, etc. – mevaqesh Mar 4 at 19:35
@mevaqesh I didn't qualify it because they didn't - it's valid across the boards. – Y ez Mar 6 at 4:31
Could you clarify this point? Especially with pertinent citations? – mevaqesh Mar 6 at 6:01
Interesting, but why would anyone consider each loci a separate piece of evidence, and not the whole sum of the collection? – avi Mar 6 at 7:14
It would be like saying you can't identify a person by their face, because the nose looks like many noses, and the eyes look like many people's eyes etc.. – avi Mar 6 at 7:20

Scientists usually throw the number one trillion in regards to how many possible distinct dna possibilities for humans.

This would mean that statistically one in 140 people has a "dna twin".

Given that testing relies on the words of pasul eidos, the fact that usually they only have a partial sample and the reality that paternity tests rely on similarity as opposed to "identicality"; the question should be why some do rely on DNA evidence

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burupark, welcome to Mi Yodeya, and thanks very much for this analysis! You could make it much more valuable by citing sources for, and more clearly explaining what you mean by, your various points about the nature of DNA testing. To begin with, 1) What does "DNA twin" mean, and how does your second statement follow from the first? 2) What does "similarity" mean, statistically? Also, please consider registering your account, which will give you access to more of the site's features. – Isaac Moses Jun 29 '12 at 17:35
Can you explain how 1 trillion possible combinations, and the existence of less than 8 billion people, means that 1 in 140 people have a "dna twin"? 1 trillion is 1,000,000,000,000, 8 billion is 8,000,000,000. Some estimate the total number of humans to have ever lived combined, to be 108 Billion. That's still 892 Billion possible combinations that have not be attempted. – avi Jun 30 '12 at 18:03
@avi I'm guessing it has to do with this en.wikipedia.org/wiki/Birthday_problem – Double AA Jul 1 '12 at 6:01
@DoubleAA Even if you assume you only need 15% of the total number of combinations to get a duplicate, that's still 150 Billion. No where close to 1 in 140. – avi Jul 1 '12 at 6:17
Thanks for your comments. By DNA twin, I meant simply a second person with the same DNA. I used the word similiarity to describe 2 DNA strings that are a partial match (I believe this is the way they refer to it in court). Since a father and son never have identical DNA the tests are checking if some of the 13 LOCI (locations) are matches. Thank you for reference to the birthday problem the site below mentions the problems before halacha, but no idea if hes right councilforresponsiblegenetics.org/pageDocuments/H4T5EOYUZI.pdf – burupark Jul 10 '12 at 17:17

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