Using Imaging to Identify Deceit: Scientific and Ethical Questions

Chapter 6: Lie Detection in the Courts: The Vain Search for the Magic Bullet

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Emilio Bizzi, Steven E. Hyman, Marcus E. Raichle, Nancy Kanwisher, Elizabeth Anya Phelps, Stephen J. Morse, Walter Sinnott-Armstrong, Jed S. Rakoff, and Henry T. Greely

Jed S. Rakoff

The detection of truth is at the heart of the legal process.1 The purpose of a trial, in particular, is to resolve the factual disputes on which a case turns, and the trial culminates with the rendering of a “verdict,” which in Latin means “to state the truth.”

Given the common tendency of witnesses to exaggerate, to embroider, and, frankly, to lie,2 how does a fact finder determine the truth? Particularly in the adversarial system of justice common to the Anglo-American tradition, truth is revealed, and lying detected, by exposing witnesses to cross-examination; that is, to tough questioning designed to test the consistency and credibility of the witness’s story. John Henry Wigmore, known to most lawyers as the most profound expositor of the rules of evidence in the history of law, famously described cross-examination as “the greatest legal engine ever invented for the discovery of truth.”3 And while not everyone is specially trained in the art of cross-examination, common experience—whether it be of parents questioning children, of customers questioning salespersons, or of reporters questioning politicians—suggests that nothing exposes fabrication like a good tough questioning.

But no one supposes that cross-examination is a perfect instrument for detecting the truth. For that matter, there probably has never been a scientific study of how effective cross-examination is in detecting lies, and I am not even sure how such a study could be devised. In any event, people have always sought for some simpler, talismanic way of separating truth from falsehood, and, relatedly, of determining guilt or innocence.

Thus, in medieval times, an accused who protested his innocence was put to such tests as the ordeal by water, in which he was bound and thrown in the river. The accused who had falsely protested his innocence would be rejected by the water and would float, whereas the honest accused would be received by the water and immediately sink. Usually he was fished out before he drowned; but, if the rescue came too late, he at least died in a state of innocence. If one accepted the basic religious theories on which these tests were premised, the tests were infallible.

As the middle ages gave way to the Renaissance, the faith-based methods of the ordeals were replaced by a more up-to-date, empirical method for determining the truth: torture. Although some men and women, in their perversity, might deny the evil of which their accusers were certain they were guilty, infliction of sufficient pain would lead them to admit, mirabile dictu, exactly what the accusers had suspected.

After a while, however, it became increasingly obvious that torture was leading to too many “false positives,” and more accurate methods were sought. In his treatise on evidence, Wigmore contends that cross-examination was originally developed as an alternative to torture.4 Today, of course, it is inconceivable that anyone would recommend the use of torture.

From the seventeenth century onward, cross-examination, for all its limitations, seemed to be the best the legal system had to offer as a means of determining the truth. In the late nineteenth century, every schoolchild knew the story of how Abe Lincoln, in defending a man accused of murder, carefully questioned the prosecution’s eyewitness as to how he was able to see the accused commit the murder in the dead of night and, when the witness said it was because there was a full moon, produced an almanac showing that on that night the moon was but a sliver. As Lincoln elsewhere said, “you can’t fool all of the people all of the time”—at least not when someone is around to ask the hard questions.

But the late nineteenth century also witnessed the growing belief that all areas of inquiry would ultimately yield to the power of science. It was just a matter of time before an allegedly “scientific” instrument for detecting lies was invented, namely, the polygraph.

The truth is that the polygraph is not remotely scientific. The theory of the polygraph—itself largely untested—is that someone who is consciously lying feels anxiety, and that that anxiety, in turn, is manifested by an increase in respiration rate, pulse rate, blood pressure, and sweating. Common experience suggests many possible flaws in this theory: more practiced liars might feel little anxiety about lying; taking a lie detector test might itself generate anxiety; sweating, pulse rate, blood pressure, and respiration rate are commonly affected by all sorts of other conditions, both external and internal; and so forth. One might hypothesize, therefore, that polygraph tests, while they might be better than pure chance in separating truth tellers from liars— after all, some people might fit the theory—would nevertheless have a high rate of error. As Marcus E. Raichle discusses elsewhere in this volume, that is precisely what the National Academies (NAS), which in 2002 reviewed the evidence on polygraph reliability, concluded. The NAS also concluded that polygraph testing has “weak scientific underpinnings”5 and that “belief in its accuracy goes beyond what the evidence suggests.”6

Not all experts agree. Reviewing the literature in 1998, the Supreme Court of the United States concluded that “the scientific community remains extremely polarized about the reliability of polygraph techniques,” with some studies concluding that polygraphs are no better than chance at detecting lies and, at the other extreme, one study concluding that polygraph results are accurate about 87 percent of the time. But even a 13 percent error rate is a high number when you are dealing with something as important as determining a witness’s credibility, let alone determining whether he or she is guilty or innocent of a crime.

Moreover, all these error-rate statistics are suspect because the scientific community is nowhere close to agreeing on how one properly establishes the base measure for determining the reliability of the polygraph. To devise an experiment in which one set of subjects is told to lie and the other set of subjects is told to tell the truth is one thing; to recreate the real-life conditions that would allow for a true test of the polygraph is quite something else. Whether any sound basis exists on which one can assert anything useful about the reliability or unreliability of the polygraph is uncertain.

Courts, being conservative and skeptical by nature, have largely tended to exclude polygraph evidence. But that has not stopped the government, the military, some private industry, and much of the public generally from accepting the polygraph as reliable—so great is the desire for a “magic bullet” that can instantly distinguish truth from falsehood.

Even the courts, while excluding polygraph evidence from the courtroom, have sometimes approved its use by the police on the cynical basis that it really does not matter whether the polygraph actually detects lying, so long as people believe that it does: if a subject believes that a polygraph actually works, he or she will be motivated to tell the truth and “confess.” The hypocrisy of this argument is staggering: the argument, in effect, is that even if the truth is that polygraph tests are, at best, error-prone, the police and other authorities should lie to people and encourage them to believe that the tests are highly accurate because this lie will encourage people to tell the truth.

Even on these terms, moreover, experience in my own courtroom suggests that the use of polygraphs is much more likely to cause mischief, or worse, than to be beneficial. Let me give just one example. The Millenium Hotel is situated next to Ground Zero. A few weeks after the attack on the Twin Towers, hotel employees were allowed back into the hotel to recover the belongings of the guests who had had to flee the premises on September 11, and one of the hotel’s security guards reported to the Federal Bureau of Investigation (FBI) that he had found in the room safe on the fiftieth floor, in a room occupied by a man named Abdullah Higazy, a copy of the Koran and a pilot’s radio of the kind used to guide planes from the ground. The FBI quickly discovered that Higazy was a former member of the Egyptian Air Force now resident in Brooklyn, but when they questioned him, he denied ever having a pilot’s radio. Hypothesizing that he was lying to cover up his use of the radio to guide the terrorist pilots to the Twin Towers, the FBI arrested Higazy and brought him before me on a material witness warrant. At the hearing, Higazy repeatedly asked to be given a polygraph test to establish that the radio was not his. I explained to him that polygraph tests were too unreliable to be admitted in court. Nevertheless, after the hearing, Higazy, over his own lawyer’s recommendation, asked the FBI to give him a polygraph test.

The FBI brought Higazy, alone, into the polygraph testing room, explaining that his lawyer could not be present because it would upset the balance of this “delicate” test. Over the next three hours, the FBI agent administering the test repeatedly told Higazy that he was not being truthful. Finally, Higazy, by now hysterical, blurted out that maybe the radio really was his. At that point, the FBI stopped the test and told the lawyer that Higazy had confessed and would be charged, at a minimum, with making false statements to the FBI and possibly with aiding and abetting the attack on the Twin Towers, a capital offense. The next day, based on the prosecutor’s flat statement that Higazy had confessed, I ordered Higazy detained without bail and he was shortly thereafter formally charged with lying to the FBI.

Three days later, an American Airlines pilot contacted the Millennium Hotel and asked if he could get back the pilot’s radio he had left there on September 11. It quickly developed that the radio was, indeed, his and had never been in Higazy’s room or possession. The Millennium security guard had made up the whole story about finding the radio in Higazy’s room, apparently because he wanted revenge for 9/11 on anyone who had Arab ancestry. The government dropped the charges against Higazy and prosecuted the security guard instead, who pled guilty to lying to the FBI.

For my part, I ordered an investigation by the government into the circumstances of the FBI’s polygraph testing, the result of which was a report assuring me that the manner and mode of Higazy’s polygraph examination was consistent with standard FBI practice. I am not sure whether this means that the FBI really believes in its polygraph results, despite their inaccuracy, or whether the FBI simply uses the façade of polygraph testing to try to elicit confessions. Either way, but for a near miracle, Mr. Higazy might likely now either be rotting in prison or facing execution.

Why have I spent so much time describing the evils of polygraphs, when the primary topic of this volume is the brave new world of brain scanning? I believe that many of the same evils are likely to result from the use of brain scanning to detect lies unless we are very, very careful. If anything, the potential for mischief is even greater, because while polygraphy was largely developed by technicians, brain scanning as a means of detecting lies is said to be the product of studies by honest-to-goodness real-life neuroscientists.

But the credentials of the scientists should not obscure the shakiness of the science.7 A basic problem with both polygraphy and brain scanning to detect lying is that no established standard exists for defining what willful deception is, let alone how to establish a base measure against which the validity and reliability of any lie-detection technique can be evaluated. What exists at this point are imaging technologies that show us patterns of activity or other events in the brain that are hypothesized to correlate with various mental states. Not one of these hypotheses has been subjected to the kind of rigorous testing that would establish its validity.

That, however, has not stopped several commercial enterprises from offering brain scanning as a purportedly scientific lie-detection technique that law enforcement agencies, private businesses, and even courts should utilize. The mere fact that evidence is proffered by someone with scientific credentials does not begin to satisfy the conditions for its admissibility in court.

In the case of the federal courts, the admissibility of expert testimony is governed by Rule 702 of the Federal Rules of Evidence. Rule 702 provides that

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training or education may testify thereto in the form of an opinion of otherwise, if(1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

Though every case must be assessed on its individual merits, brain scanning as a means of assessing credibility likely suffers from several defects that would render such evidence inadmissible under Rule 702 as it has been interpreted in the federal courts.

First, and perhaps most fundamentally, there is no commonly accepted theory of how brain patterns evidence lying: in the absence of such a theory, all that is being shown, at best, is the presence or absence of certain brain patterns that allegedly correlate with some hypothesized accompaniment of lying, such as anxiety. But no scientific evidence has shown either that lying is always accompanied by anxiety or that anxiety cannot be caused by a dozen other factors that cannot be factored out.

Second, the theories that have been proposed have not been put to the test of falsifiability, which, if one accepts (as the Supreme Court does) a Popper-like view of science, is the sine qua non of assessing scientific validity and reliability.

Third, no standard way exists of defining what lying is, let alone how to test for it. The law recognizes many kinds of lies, ranging from “white lies” and “puffing” to affirmative misstatements, actionable half-truths, and material omissions. Brain scans cannot yet come close to distinguishing between these different kinds of lying. Yet the differences are crucial in almost any case: a little white lie is altogether different, in the eyes of the law and of common sense, from an intentional scheme to defraud. Nothing in the brain-scan approach to lie detection even attempts to make such distinctions. And what might a brain scan be predicted to show in the case of a lie by omission; that is, the person whose statements are truthful as far as they go but who conceals a material fact that puts an entirely different perspective on what is being said? In my experience, these are the most common kinds of lies in court, and they are revealed only by a good cross-examination.

For these and other reasons, brain-scanning tests of credibility might well fail to meet the tests for admissibility under Rule 702, both because the brain-scanning tests lack scientific reliability and because they are unlikely to be useful to the jury. Additionally, even evidence that qualifies for admission under Rule 702 may be excluded under Rule 403 of the Federal Rules of Evidence, which provides that “Although relevant, evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by considerations of undo delay, waste of time, or needless presentation of cumulative evidence.” Brain-scanning evidence, precisely because it holds itself out as truly scientific, is likely to have a much greater impact on the trier of fact than its limited theoretical and experimental bases can fairly support; it therefore might also be excluded under Rule 403.

A time may come when some sort of brain-scanning technique will be developed that will actually show the act of willful lying with enough accuracy and predictability as to meet both the requirements of law and the even more rigorous standards of accepted science. But long before that occurs, claims will be made—some even by reputable scientists whose enthusiasm for their own studies has overwhelmed all caution—that science has developed brain-scan lie detectors that are perfectly accurate, when in fact they are not. Indeed, some such claims are already being made. The desire for a magic bullet that exposes lies is so strong that many people will be persuaded that, indeed, brain scans can achieve what polygraphers have long claimed but, in my view, wholly failed to achieve. The result will be an abdication of the difficult work of actually detecting lies through cross-examination in favor of quasi-scientific tests that substitute the façade of scientific certainty for the actuality of truth.


1. This article, based on remarks made at the American Academy of Arts and Sciences’s conference on February 2, 2007, presents the author’s personal views and does not reflect how he might decide any legal issue in any given case.

2. In my remarks on which this article is based, I estimated that 90 percent of all material trial witnesses knowingly lie in some respect (though not always materially). This estimate is based on my experience as a trial judge for the past twelve years and as a trial lawyer for the twenty-five years previous to that and has no scientific study to back it up. Possibly I am too harsh: perhaps the percentage of material witnesses who consciously lie in some respect is as low as, say, 89 percent.

3. John Henry Wigmore, Evidence In Trials At Common Law (Chadbourn Edition, 1974), Vol. 5, p. 32.

4. Ibid., p. 32ff.

5. The National Academies, National Research Council, “Polygraph Testing Too Flawed for Security Screening,” October 8, 2002, p. 2.

6. The National Academies, National Research Council, Committee to Review the Scientific Evidence on the Polygraph, The Polygraph and Lie Detection (2003), p. 7.

7. For a more expert discussion of the limitations of brain scanning as a truth-detection device, see the articles by Elizabeth Phelps and Nancy Kanwisher elsewhere in this volume.