False Alarm: Does Halacha Accept the Polygraph?

Adapted from the writings of Dayan Yitzhak Grossman

April 23, 2026

From Undark Magazine:

Scientists, the press, and the justice system have also derided polygraphs (lie detector devices) as inaccurate. Research has suggested that the physiological signals they pick up are prone to false positives and not enough true positives. Questions about their scientific validity are, in fact, why they’re not admissible in most US courts…

Given those doubts, researchers and corporations are trying to find more reliable and modern ways to detect deception. Their methods—which span everything from monitoring involuntary eye behaviors to brain activity—also aren’t perfect. And some researchers question whether such an endeavor is even possible…

Today, polygraphs measure the same changes (to pulse, blood pressure, and respiration) Larson did, in addition to measuring how well the skin conducts electricity, a proxy for sweating. Typically, examiners interview their subjects ahead of time and gather baseline numbers on their physiological ticking. The measured exam, meanwhile, includes neutral control questions as well as questions relevant to whatever the investigators are seeking the truth about. The idea is that if someone is lying, their physiology will show stress compared to their truthful baseline. Their heart rate will elevate; they will sweat more; their blood pressure will increase; they will breathe faster. An examiner would see those spikes in the graphs of each metric, then analyze whether a spike—especially in all the different measurements at once—indicates a lie.

But, according to numerous studies, polygraphs cannot reliably detect lying, or truth-telling, and their use in the justice and employment systems is regulated due to those problems with scientific reliability. A landmark 2003 report from the National Academies of Sciences, Engineering, and Medicine found the quality of research about polygraphy to be low, the theoretical explanation of how it functions (and why it detects lying, and not, say, nervousness) to be inadequate, the rate of false positives to be unacceptable, and the rate of false negatives to be a risk…

Given these drawbacks, scientists are investigating whether better options for lie detection exist, more grounded in evidence and technologies that weren’t available when the polygraph was invented. Some of those options aren’t more accurate than the traditional method, but others are showing more promise.

One avenue doesn’t require throwing out polygraphs as a tool, but simply relying less on humans to be the sole arbiters of their results. A 2023 paper in Nature’s Scientific Reports, for instance, described machine-learning models created by the study authors to give a second opinion on human examiners’ conclusions…[1]

A number of poskim and dayanim of the last century addressed polygraphs; this article and a sequel discuss their various attitudes toward current polygraph technology and consider how they would apply to newer methods of lie detection. This article surveys the positions of several poskim that say polygraph examinations may be relied upon at least to some extent, and the next one will iy”H survey the positions of poskim that disagree.

One rule of evidence in the Gemara that is often cited in discussions of polygraphs is din merumeh (fraudulent proceedings):

From where is it derived that if the judge knows that the proceedings are fraudulent, he should not say to himself: Since the witnesses have testified, I will decide the case based on their testimony, and a chain (i.e., responsibility for the miscarriage of justice) will hang around the necks of the witnesses? The Torah says: “Distance yourself from falsehood.”[2]

The Eimek Halacha (R’ Yehoshua Baumol) was asked about the halachic validity of the lie detector and of “the new invention called ‘truth serum,’ whose effect is that the subject finds himself unable to lie and to think otherwise, but only to speak the truth.”[3] His answer takes for granted that polygraph results constitute umdena demuchach (a compelling assessment), so he maintains that if the machine indicates that witnesses are lying, their testimony is rejected under din merumeh:

If they accepted the testimony in a case of din merumeh and the machine’s needle is vibrating, which indicates that the witnesses are lying, it appears clear that their testimony is void. Although the machine is not absolute proof that they are lying, only an assessment that they are not telling the truth, and it is in the category of umdena demuchach, nevertheless we may certainly not rule based on their testimony, as Tosfos in Sanhedrin 32 s.v. Kan bedin merumeh wrote… It is evident from the words of Tosfos that where we know that it is fraudulent, their testimony is void. Via the umdena demuchach of the machine, the testimony is certainly in the category of “we know that it is fraudulent,” so we may certainly not rule based on the witnesses, and this is clear.

Rav Baumol nevertheless rules that for various reasons, batei din should not routinely subject witnesses to polygraph testing in either criminal or civil cases, but they are certainly permitted to do so in a case of din merumeh.

The Debrecener Rav (R’ Moshe Stern, the Be’er Moshe) rules similarly that if a polygraph indicates that witnesses are lying, their testimony is rejected. But contrary to Rav Baumol, he maintains that the evidentiary weight of a polygraph test is not umdena demuchach, only ordinary umdena. Still, he considers this sufficient basis to reject the testimony.[4]

R’ Moshe Feinstein endorses a very different case of reliance upon a polygraph examination:

A sick person who is not able to speak or even to nod his head or gesticulate in any other manner, and we discern his intention to divorce his wife via the polygraph machine that displays his heartbeat and similar physiological signs, this, too, suffices to indicate his intention to divorce. So if at that time they examined him with respect to other matters and they saw that he answered correctly, with knowledge and understanding in his responses, and they immediately asked him if he wishes to give a get to his wife, we may rely on this…[5]

[1]Sarah Scoles. Polygraphs Aren’t Very Accurate. Are There Better Options? Undark Magazine.

[2]Shmos 23:7, Shvuos 30b-31a.

[3]Shu”t Eimek Halacha cheilek 2 siman 14.

[4]Shu”t Be’er Moshe cheilek 7, Kuntres Electric cheilek 2 siman 79, and cf. siman 80.

[5]Shu”t Igros Moshe E.H. cheilek 4 siman 98.