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The Polygraph and Lie Detection 7 Uses of Polygraph Tests The available evidence indicates that in the context of specific-incident investigation and with inexperienced examinees untrained in countermeasures, polygraph tests as currently used have value in distinguishing truthful from deceptive individuals. However, they are far from perfect in that context, and important unanswered questions remain about polygraph accuracy in other important contexts. No alternative techniques are available that perform better, though some show promise for the long term. The limited evidence on screening polygraphs suggests that their accuracy in field use is likely to be somewhat lower than that of specific-incident polygraphs. This chapter discusses the policy issues involved in using an imperfect diagnostic test such as the polygraph in real-life decision making, particularly in national security screening, which presents very difficult tradeoffs between falsely judging innocent employees deceptive and leaving major security threats undetected. We synthesize what science can offer to inform the policy decisions, but emphasize that the choices ultimately must depend on a series of value judgments incorporating a weighting of potential benefits (chiefly, deterring and detecting potential spies, saboteurs, terrorists, or other major security threats) against potential costs (such as of falsely accusing innocent individuals and losing potentially valuable individuals from the security related workforce). Cost-benefit tradeoffs like this vary with the situation. For example, the benefits are greater when the security threat being investigated is more serious; the costs are greater when the innocent individuals who might be
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The Polygraph and Lie Detection accused are themselves vital to national security. For this reason, tradeoff decisions are best made by elected officials or their designees, aided by the principles and practices of behavioral decision making. We first summarize what scientific analysis can contribute to understanding the tradeoffs involved in using polygraph tests in security screening. (These tests almost always use the comparison question or relevant-irrelevant formats because concealed information tests can only be used when there are specific pieces of information that can form the basis for relevant questions.) We then discuss possible strategies for making the tradeoffs more attractive by improving the accuracy of lie detection— either by making polygraph tests more accurate or by combining them with other sources of information. We also briefly consider the legal context of policy choices about the use of polygraph tests in security screening. TRADEOFFS IN INTERPRETATION The primary purpose of the polygraph test in security screening is to identify individuals who present serious threats to national security. To put this in the language of diagnostic testing, the goal is to reduce to a minimum the number of false negative cases (serious security risks who pass the diagnostic screen). False positive results are also a major concern: to innocent individuals who may lose the opportunity for gainful employment in their chosen professions and the chance to help their country and to the nation, in the loss of valuable employees who have much to contribute to improved national security, or in lowered productivity of national security organizations. The prospect of false positive results can also have this effect if employees resign or prospective employees do not seek employment because of polygraph screening. As Chapter 2 shows, polygraph tests, like any imperfect diagnostic tests, yield both false positive and false negative results. The individuals judged positive (deceptive) always include both true positives and false positives, who are not distinguishable from each other by the test alone. Any test protocol that produces a large number of false positives for each true positive, an outcome that is highly likely for polygraph testing in employee security screening contexts, creates problems that must be addressed. Decision makers who use such a test protocol might have to decide to stall or sacrifice the careers of a large number of loyal and valuable employees (and their contributions to national security) in an effort to increase the chance of catching a potential security threat, or to apply expensive and time-consuming investigative resources to the task of identifying the few true threats from among a large pool of individuals who had positive results on the screening test.
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The Polygraph and Lie Detection Quantifying Tradeoffs Scientific analysis can help policy makers in such choices by making the tradeoffs clearer. Three factors affect the frequency of false negatives and false positives with any diagnostic test procedure: its accuracy (criterion validity), the threshold used for declaring a test result positive, and the base rate of the condition being diagnosed (here, deception about serious security matters). If a diagnostic procedure can be made more accurate, the result is to reduce both false negatives and false positives. With a procedure of any given level of accuracy, however, the only way to reduce the frequency of one kind of error is by adjusting the decision threshold—but doing this always increases the frequency of the other kind of error. Thus, it is possible to increase the proportion of guilty individuals caught by a polygraph test (i.e., to reduce the frequency of false negatives), but only by increasing the proportion of innocent individuals whom the test cannot distinguish from guilty ones (i.e., frequency of false positives). Decisions about how, when, and whether to use the polygraph for screening should consider what is known about these tradeoffs so that the tradeoffs actually made reflect deliberate policy choices. Tradeoffs between false positives and false negatives can be calculated mathematically, using Bayes’ theorem (Weinstein and Fineberg, 1980; Lindley, 1998). One useful way to characterize the tradeoff in security screening is with a single number that we call the false positive index: the number of false positive cases to be expected for each deceptive individual correctly identified by a test. The index depends on the accuracy of the test; the threshold set for declaring a test positive; and the proportion, or base rate, of individuals in the population with the condition being tested (deception, in this case). The specific mathematical relationship of the index to these factors, and hence the exact value for any combination of accuracy (A), threshold, and base rate, depends on the shape of the receiver operating characteristic (ROC) curve at a given level of accuracy, although the character of the relationship is similar across all plausible shapes (Swets, 1986a, 1996:Chapter 3). Hence, for illustrative purposes we assume that the ROC shapes are determined by the simplest common model, the equivariance binormal model.1 Because this model, while not implausible, was chosen for simplicity and convenience, the numerical results below should not be taken literally. However, their orders of magnitude are unlikely to change for any alternative class of ROC curves that would be credible for real-world polygraph test performance, and the basic trends conveyed are inherent to the mathematics of diagnosis and screening. Although accuracy, detection threshold, and base rate all affect the
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The Polygraph and Lie Detection false positive index, these determinants are by no means equally important. Calculation of the index for diagnostic tests at various levels of accuracy, using various thresholds, and with a variety of base rates shows clearly that base rate is by far the most important of these factors. Figure 7-1 shows the index as a function of the base rate of positive (e.g., deceptive) cases for three thresholds for a diagnostic test with A = 0.80. It illustrates clearly that the base rate makes more difference than the threshold across the range of thresholds presented. Figure 7-2 shows the index as a function of accuracy with the threshold held constant so that the diagnostic test’s sensitivity (percent of deceptive individuals correctly identified) is 50 percent. It illustrates clearly that base rate makes more difference than the level of accuracy across the range of A values represented. Figures 7-1 and 7-2 show that the tradeoffs involved in relying on a diagnostic test such as the polygraph, represented by the false positive index values on the vertical axis, are sharply different in situations with high base rates typical of event-specific investigations, when all examinees are identified as likely suspects, and the base rate is usually above 10 percent, than in security screening contexts, when the base rate is normally very low for the most serious infractions. The false positive index is FIGURE 7-1 Comparison of the false positive index and base rate for three sensitivity values of a polygraph test protocol with an accuracy index (A) of 0.80.
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The Polygraph and Lie Detection FIGURE 7-2 Comparison of the false positive index and base rate for four values of the accuracy index (A) for a polygraph test protocol with threshold set to correctly identify 50 percent of deceptive examinees. about 1,000 times higher when the base rate is 1 serious security risk in 1,000 than it is when the base rate is 1 in 2, or 50 percent. The index is also affected, though less dramatically, by the accuracy of the test procedure: see Figure 7-2. (Appendix I presents the results of calculations of false positive indexes for various levels of accuracy, base rates, and thresholds for making a judgment of a positive test result.) With very low base rates, such as 1 in 1,000, the false positive index is quite large even for tests with fairly high accuracy indexes. For example, a test with an accuracy index of 0.90, if used to detect 80 percent of major security risks, would be expected to falsely judge about 200 innocent people as deceptive for each security risk correctly identified. Unfortunately, polygraph performance in field screening situations is highly unlikely to achieve an accuracy index of 0.90; consequently, the ratio of false positives to true positives is likely to be even higher than 200 when this level of sensitivity is used. Even if the test is set to a somewhat lower level of sensitivity, it is reasonable to expect that each spy or terrorist that might be correctly identified as deceptive by a polygraph test of the accuracy actually achieved in the field would be accompanied by at least hundreds of nondeceptive examinees mislabeled as deceptive. The spy or terrorist would be indistinguishable from these false positives by poly-
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The Polygraph and Lie Detection graph test results. The possibility that deceptive examinees may use countermeasures makes this tradeoff even less attractive. It is useful to consider again the tradeoff of false positives versus false negatives in a manner that sets an upper bound on the attractiveness of the tradeoff (see Table 2-1, p. 48). The table shows the expected outcomes of polygraph testing in two hypothetical populations of examinees, assuming that the tests achieve an accuracy index of 0.90, which represents a higher level of accuracy than can be expected of field polygraph testing. One hypothetical population consists of 10,000 criminal suspects, of whom 5,000 are expected to be guilty; the other consists of 10,000 employees in national security organizations, of whom 10 are expected to be spies. The table illustrates the tremendous difference between these two populations in the tradeoff. In the hypothetical criminal population, the vast majority of those who “fail” the test (between 83 and 98 percent in these examples) are in fact guilty. In the hypothetical security screening population, however, because of the extremely low base rate of spies, the vast majority of those who “fail” the test (between 95 and 99.5 percent in these examples) are in fact innocent of spying. Because polygraph testing is unlikely to achieve the hypothetical accuracy represented here, even these tradeoffs are overly optimistic. Thus, in the screening examples, an even higher proportion than those shown in Table 2-1 would likely be false positives in actual practice. We reiterate that these conclusions apply to any diagnostic procedure that achieves a similar level of accuracy. None of the alternatives to the polygraph has yet been shown to have greater accuracy, so these upper bounds apply to those techniques as well. Tradeoffs with “Suspicious” Thresholds If the main objective is to screen out major security threats, it might make sense to set a “suspicious” threshold, that is, one that would detect a very large proportion of truly deceptive individuals. Suppose, for instance, the threshold were set to correctly identify 80 percent of truly deceptive individuals. In this example, the false positive index is higher than 100 for any base rate below about 1 in 500, even with A = 0.90. That is, if 20 of 10,000 employees were serious security violators, and polygraph tests of that accuracy were given to all 10,000 with a threshold set to correctly identify 16 of the 20 deceptive employees, the tests would also be expected to identify about 1,600 of the 9,980 good security risks as deceptive.2 Another way to think about the effects of setting a threshold that correctly detects a very large proportion of deceptive examinees is in terms of the likelihood that an examinee who is judged deceptive on the
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The Polygraph and Lie Detection test is actually deceptive. This probability is the positive predictive value of the test. If the base rate of deceptive individuals in a population of examinees is 1 in 1,000, an individual who is judged deceptive on the test will in fact be nondeceptive more than 199 times out of 200, even if the test has A = 0.90, which is highly unlikely for the polygraph (the actual numbers of true and false positives in our hypothetical population are shown in the right half of part a of Table 2-1). Thus, a result that is taken as indicating deception on such a test does so only with a very small probability. These numbers contrast sharply with their analogs in a criminal investigation setting, in which people are normally given a polygraph test only if they are suspects. Suppose that in a criminal investigation the polygraph is used on suspects who, on other grounds, are estimated to have a 50 percent chance of being guilty. For a test with A = 0.80 and a sensitivity of 50 percent, the false positive index is 0.23 and the positive predictive value is 81 percent. That means that someone identified by this polygraph protocol as deceptive has an 81 percent chance of being so, instead of the 0.4 percent (1 in 250) chance of being so if the same test is used for screening a population with a base rate of 1 in 1,000.3 Thus, a test that may look attractive for identifying deceptive individuals in a population with a base rate above 10 percent looks very much less attractive for screening a population with a very low base rate of deception. It will create a very large pool of suspect individuals, within which the probability of any specific individual being deceptive is less than 1 percent—and even so, it may not catch all the target individuals in the net. To put this another way, if the polygraph identifies 100 people as indicating deception, but only 1 of them is actually deceptive, the odds that any of these identified examinees is attempting to deceive are quite low, and it would take strong and compelling evidence for a decision maker to conclude on the basis of the test that this particular examinee is that 1 in 100 (Murphy, 1987). Although actual base rates are never known for any type of screening situation, base rates can be given rough bounds. In employee screening settings, the base rate depends on the security violation. It is probably far higher for disclosure of classified information to unauthorized individuals (including “pillow talk”) than it is for espionage, sabotage, or terrorism. For the most serious security threats, the base rate is undoubtedly quite low, even if the number of major threats is 10 times as large as the number of cases reported in the popular press, reflecting both individuals caught but not publicly identified and others not caught. The one major spy caught in the FBI is one among perhaps 100,000 agents who have been employed in the bureau’s history. The base rate of major security threats in the nation’s security agencies is almost certainly far less than 1 percent.
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The Polygraph and Lie Detection Appendix I presents a set of curves that allow readers to estimate the false positive index and consider the implied tradeoff for a very wide range of hypothesized base rates of deceptive examinees and various possible values of accuracy index for the polygraph testing, using a variety of decision thresholds. It is intended to help readers consider the tradeoffs using the assumptions they judge appropriate for any particular application. Thus, using the polygraph with a “suspicious” threshold so as to catch most of the major security threats creates a serious false-positive problem in employee security screening applications, mainly because of the very low base rate of guilt among those likely to be screened. When the base rate is one in 1,000 or less, one can expect a polygraph test with a threshold that correctly identifies 80 percent of deceptive examinees to incorrectly classify at least 100 nondeceptive individuals as deceptive for each security threat correctly identified. Any diagnostic procedure that implicates large numbers of innocent employees for each major security violator correctly identified comes with a variety of costs. There is the need to investigate those implicated, the great majority of whom are innocent, as well as the issue of the civil liberties of innocent employees caught by the screen. There is the potential that the screening policy will create anxiety that decreases morale and productivity among the employees who face screening. Employees who are innocent of major security violations may be less productive when they know that they are being tested routinely with an instrument that produces a false positive reading with non-negligible probability and when such a reading can put them under suspicion of disloyalty. Such effects are most serious when the deception detection threshold is set to detect threats with a reasonably high probability (above 0.5), because such a threshold will also identify considerable numbers of false positive outcomes among innocent employees. And there is the possibility that people who might have become valued employees will be deterred from taking positions in security agencies by fear of false positive polygraph results. To summarize, the performance of the polygraph is sharply different in screening and in event-specific investigation contexts. Anyone who believes the polygraph “works” adequately in a criminal investigation context should not presume without further careful analysis that this justifies its use for security screening. Each application requires separate evaluation on its own terms. To put this another way, if the polygraph or any other technique for detecting deception is more accurate than guess-work, it does not necessarily follow that using it for screening is better than not using it because a decision to use the polygraph or any other imperfect diagnostic technique must consider its costs as well as its benefits. In the case of polygraph screening, these costs include not only the
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The Polygraph and Lie Detection civil liberties issues that are often debated in the context of false positive test results, but also two types of potential threats to national security. One is the false sense of security that may arise from overreliance on an imperfect screen: this could lead to undue relaxation of other security efforts and thus increase the likelihood that serious security risks who pass the screen can damage national security. The other cost is associated with damage to the national security that may result from the loss of essential personnel falsely judged to be security risks or deterred from employment in U.S. government security agencies by the prospect of false-positive polygraph results. Tradeoffs with “Friendly” Thresholds The discussion to this point assumes that policy makers will use a threshold such that the probability of detecting a spy is fairly high. There is, however, another possibility: they may decide to set a “friendly” threshold, that is, one that makes the probability of detecting a spy quite low. To the extent that testing deters security violations, such a test might still have utility for national security purposes. This deterrent effect is likely to be stronger when there is at least a certain amount of ambiguity concerning the setting of threshold. (If it were widely known that no one “failed” the test, its deterrent effect would be considerably lessened.) It is possible, however, to set a threshold such that almost no one is eventually judged deceptive, even though a fair number undergo additional investigation or testing. There is a clear difference between employment in the absence of security screening tests, a situation lacking in deterrent value against spies, and employment policies that include screening tests, even if screening identifies few if any spies. Our meetings with various federal agencies that use polygraph screening suggest that different agencies set thresholds differently, although the evidence we have is anecdotal. Several agencies’ polygraph screening programs, including that of the U.S. Department of Energy, appear to adopt fairly “friendly” effective thresholds, judged by the low proportion of polygraph tests that show significant response. The net result is that these screening programs identify a relatively modest number of cases to be investigated further, with few decisions eventually being made that the employee has been deceptive about a major security infraction. There are reasons of utility, such as possible deterrent effects, that might be put forward to justify an agency’s use of a polygraph screening policy with a friendly threshold, but such a polygraph screening policy will not identify most of the major security violators. For example, the U.S. Department of Defense (2001:4) reported that of 8,784 counterintelli-
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The Polygraph and Lie Detection gence scope polygraph examinations given, 290 (3 percent) individuals gave “significant responses and/or provided substantive information.” The low rate of positive test results suggests that a friendly threshold is being used, such that the majority of the major security threats who took the test would “pass” the screen.4 On April 4, 2002, the director of the Federal Bureau of Investigation (FBI) was quoted in the New York Times as saying that “less than 1 percent of the 700” FBI personnel who were given polygraph tests in the wake of the Hanssen spy case had test results that could not be resolved and that remain under investigation (Johnston, 2002). Whatever value such a polygraph testing protocol may have for deterrence or eliciting admissions of wrongdoing, it is quite unlikely to uncover an espionage agent who is not deterred and does not confess. A substantial majority of the major security threats who take such a test would “pass” the screen.5 For example, if Robert Hanssen had taken such tests three times during 15 years of spying, the chances are that, even without attempting countermeasures, he would not have been detected before considerable damage had been done. (He most likely would never have been detected unless the polygraph protocol achieved a criterion validity that we regard as unduly optimistic, such as A = 0.90.) Furthermore, if Hanssen had been detected as polygraph positive (along with a large number of non-spies, that is, false positives), he would not necessarily have been identified as a spy. There may be justifications for polygraph screening with a “friendly” threshold on the grounds that the technique may have a deterrent effect or may yield admissions of wrongdoing. However, such a screen will not identify most of the major security threats. In our judgment, the accuracy of polygraph testing in distinguishing actual or potential security violators from innocent test takers is insufficient to justify reliance on its use in employee screening in federal agencies. Although we believe it likely that polygraph testing has utility in screening contexts because it might have a deterrent effect, we were struck by the lack of scientific evidence concerning the factors that might produce or inhibit deterrence. In order to properly evaluate the costs and benefits associated with polygraph screening, research is needed on deterrence in general and, in particular, on the effects of polygraph screening on deterrence. Recent Policy Recommendations on Polygraph Screening We have great concern about the dangers that may arise for national security if federal agencies use the polygraph for security screening with an unclear or incorrect understanding of the implications of threshold-setting choices for the meaning of test results. Consider, for instance,
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The Polygraph and Lie Detection decisions that might be made on the basis of the discussion of polygraph screening in the recent report of a select commission headed by former FBI director William H. Webster (the “Webster Commission”) (Commission for the Review of FBI Security Programs, 2002). This report advocates expanded use of polygraph screening in the FBI, but does not take any explicit position on whether polygraph testing has any scientific validity for detecting deception. This stance is consistent with a view that much of the value of the polygraph comes from its utility for deterrence and for eliciting admissions. The report’s reasoning, although not inconsistent with the scientific evidence, has some implications that are reasonable and others that are quite disturbing from the perspective of the scientific evidence on the polygraph. The Webster Commission recognizes that the polygraph is an imperfect instrument. Its recommendations for dealing with the imperfections, however, address only some of the serious problems associated with these imperfections. First, it recommends increased efforts at quality control and assurance and increased use of “improved technology and computer driven systems.” These recommendations are sensible, but they do not address the inherent limitations of the polygraph, even when the best quality control and measurement and recording techniques are used. Second, it takes seriously the problem of false positive errors, noting that at one point, the U.S. Central Intelligence Agency (CIA) had “several hundred unresolved polygraph cases” that led to the “practical suspension” of the affected officers, sometimes for years, and “a devastating effect on morale” in the CIA. The Webster Commission clearly wants to avoid a repetition of this situation at the FBI. It recommends that “adverse personnel actions should not be taken solely on the basis of polygraph results,” a position that is absolutely consistent with the scientific evidence that false positives cannot be avoided and that in security screening applications, the great majority of positives will turn out to be false. It also recommends a polygraph test only for “personnel who may pose the greatest risk to national security.” This position is also strongly consistent with the science, though the commission’s claim that such a policy “minimizes the risk of false positives” is not strictly true. Reducing the number of employees who are tested will reduce the total number of false positives, and therefore the cost of investigating false positives, but will not reduce the risk that any individual truthful examinee will be a false positive or that any individual positive result will be false. That risk can only be reduced by finding a more accurate test protocol or by setting a more “friendly” threshold. Because the Webster Commission report does not address the problem of false-negative errors in any explicit way, it leaves open the possibility that federal agency officials may draw the wrong conclusions from
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The Polygraph and Lie Detection have been in practice in other areas, but the most “successful” expert systems for medical diagnosis require a substantial body of theory or empirical knowledge that link clearly identified measurable features with the condition being diagnosed (see Appendix F). For screening uses of the polygraph, it seems clear that no such body of knowledge exists. Lacking such knowledge, the serious problems that exist in deriving and adequately validating procedures for computer scoring of polygraph tests (discussed above) also exist for the derivation and validation of expert systems for combining polygraph results with other diagnostic information. Insufficient scientific information exists to support recommendations on whether or how to combine polygraph and other information in a sequential screening model. A number of psychophysiological techniques appear promising in the long run but have not yet demonstrated their validity. Some indicators based on demeanor and direct investigation appear to have a degree of accuracy, but whether they add information to what the polygraph can provide is unknown (see Chapter 6). LEGAL CONTEXT The practical use of polygraph testing is shaped in part by its legal status. Polygraph testing has long been the subject of judicial attention, much more so than most forensic technologies. In contrast, courts have only recently begun to look at the data, or lack thereof, for other forensic technologies, such as fingerprinting, handwriting identification and bite marks, which have long been admitted in court. The attention paid to polygraphs has generally led to a skeptical view of them by the judiciary, a view not generally shared by most executive branch agencies. Judicial skepticism results both from questions about the validity of the technology and doubt about its need in a constitutional process that makes juries or judges the finders of fact. Doubts about polygraph tests also arise from the fact that the test itself contains a substantial interrogation component. Courts recognize the usefulness of interrogation strategies, but hesitate when the results of an interrogation are presented as evidentiary proof. Although polygraphs clearly have utility in some settings, courts have been unwilling to conclude that utility denotes validity. The value of the test for law enforcement and employee screening is an amalgam of utility and validity, and the two are not easily separated. An early form of the polygraph served as the subject of the wellknown standard used for evaluating scientific evidence—general acceptance—announced in Frye v. United States (1923) and still used in some courts (see below). It has been the subject of a U.S. Supreme Court decision, United States v. Scheffer (1998), and countless state and federal deci-
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The Polygraph and Lie Detection sions (see Appendix E for details on the Frye case). In Scheffer, the Court held that the military’s per se rule excluding polygraphs was not unreasonable—and thus not unconstitutional—because there was substantial dispute among scientists concerning the test’s validity. Polygraphs fit the pattern of many forensic scientific fields, being of concern to the courts, government agencies and law enforcement, but largely ignored by the scientific community. A recent decision found the same to be true for fingerprinting (United States v. Plaza, 2002). In Plaza, the district court initially excluded expert opinion regarding whether a latent fingerprint matched the defendant’s print because the applied technology of the science had yet to be adequately tested and was almost exclusively reviewed and accepted by a narrow group of technicians who practiced the art professionally. Although the district court subsequently vacated this decision and admitted the evidence, the judge repeated his initial finding that fingerprinting had not been tested and was only generally accepted within a discrete and insular group of professionals. The court, in fact, likened fingerprint identification to accounting and believed it succeeded as a “specialty” even though it failed as a “science.”8 Courts have increasingly noticed that many forensic technologies have little or no substantial research behind them (see e.g., United States v. Hines  on handwriting analysis and the more general discussion in Faigman et al. ). The lack of data on regularly used scientific evidence appears to be a systemic problem and, at least partly, a product of the historical divide between law and science. Federal courts only recently began inquiring directly into the validity or reliability of proffered scientific evidence. Until 1993, the prevailing standard of admissibility was the general acceptance test first articulated in Frye v. United States in 1923. Using that test, courts queried whether the basis for proffered expert opinion is generally accepted in the particular field from which it comes. In Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993), however, the U.S. Supreme Court held that Frye does not apply in federal courts. Under the Daubert test, judges must determine whether the basis for proffered expertise is, more likely than not, valid. The basic difference between Frye and Daubert is one of perspective: courts using Frye are deferential to the particular fields generating the expertise, whereas Daubert places the burden on the courts to evaluate the scientific validity of the expert opinion. This difference of perspective has begun to significantly change the reception of the scientific approach in the court-room.9 Much of the expert opinion that has been presented as “scientific” in courts is not based on what scientists recognize as solid scientific evidence, or even, in some cases, rudimentary scientific methods and prin-
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The Polygraph and Lie Detection ciples. The polygraph is not unusual in this regard. In fact, topics such as bite mark and hair identification, fingerprinting, arson investigation, and tool mark analysis have a less extensive record of research on accuracy than does polygraph testing. Historically, the courts relied on experts in sundry fields in which the basis for the expert opinion is primarily assertion rather than scientific testing and in which the value of the expertise is measured by effectiveness in court rather than scientific demonstration of accuracy or validity. These observations raise several issues worthy of consideration. First, if the polygraph compares well with other forensic sciences, should it not receive due recognition for its relative success? Second, most forensic sciences are used solely in judicial contexts, while the polygraph is also used in employment screening: Do the different contexts in which the technique is used affect the determination of its usefulness? And third, since mainstream scientists have largely ignored forensic science, how could this situation be changed? We consider these matters in turn. Polygraph Testing as a Forensic Science Without question, DNA profiling provides the model of cooperation between science and the law. The technology was founded on basic science, and much of the early debate engaged a number of leading figures in the scientific community. Rapidly improving technology and expanded laboratory attention led to improvements in the quality of the data and the strengths of the inferences that could be drawn. Even then, however, there were controversies regarding the statistical inferences (National Research Council, 1992, 1996a). Nonetheless, from the start, judges understood the need to learn the basic science behind the technology and, albeit with certain exceptions, largely mastered both the biology and the statistics underlying the evidence. At the same time, DNA profiling might be somewhat misleading as a model for the admissibility of scientific evidence. Although some of the forensic sciences, such as fingerprinting (see Cole, 2001), started as science, most have existed for many decades outside mainstream science. In fact, many forensic sciences had their start well outside the scientific mainstream. Moreover, although essentially probabilistic, DNA profiling today produces almost certain conclusions—if a sufficient set of DNA characteristics is measured, the resulting DNA profiles can be expected to be unique, with a probability of error of one in billions or less (except for identical twins) (National Research Council, 1996a). This near certainty of DNA evidence may encourage some lawmakers’ naive view that science, if only it is good enough, will produce certain answers. (In fact, the one
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The Polygraph and Lie Detection area in which DNA profiling is least certain, laboratory error, is the area in which courts have had the most difficulty in deciding how to handle the uncertainty.) The accuracy of polygraph testing does not come anywhere near what DNA analysis can achieve. Nevertheless, polygraph researchers have produced considerable data concerning polygraph validity (see Chapters 4 and 5). However, most of this research is laboratory research, so that the generalizability of the research to field settings remains uncertain. The field studies that have been carried out also have serious limitations (see Chapter 4). Moreover, there is virtually no standardization of protocols; the polygraph tests conducted in the field depend greatly on the presumed skill of individual examiners. Thus, even if laboratory-based estimates of criterion validity are accurate, the implications for any particular field polygraph test are uncertain. Without the further development of standardized polygraph testing techniques, the gulf between laboratory validity studies and inferences about field polygraph tests will remain wide. The ambiguity surrounding the validity of field polygraphs is complicated still further by the structure of polygraph testing. Because in practice the polygraph is used as a combination of lie detector and interrogation prop, the examiner typically is privy to information regarding the examinee. While this knowledge is invaluable for questioning, it also might lead to examiner expectancies that could affect the dynamic of the polygraph testing situation or the interpretation of the test’s outcome. Thus, high validity for laboratory testing might again not be indicative of the validity of polygraphs given in the field. Context of Polygraph Testing The usefulness of polygraph test results depends on the context of the test and the consequences that follow its use. Validity is not something that courts can assess in a vacuum. The wisdom of applying any science depends on both the test itself and the application contemplated. A forensic tool’s usefulness depends on the specific nature of the test (i.e., in what situation might it apply?), the import or relevance of the test (i.e., what inferences follow from “failing” or “passing” the test?), the consequences that follow the test’s administration (e.g., denial of employment, discharge, criminal prosecution), and the objective of the test (lie detection or interrogation). A principal consideration in the applied sciences concerns the content of a test: what it does, or can be designed to, test. Concealed information polygraph tests, for example, have limited usefulness as a screening device simply because examiners usually cannot create specific questions
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The Polygraph and Lie Detection about unknown transgressions. (There may be exceptions, as in some focused screening applications, as discussed above.) The application of any forensic test, therefore, is limited by the test’s design and function. Similarly, the import of the test itself must be considered. For instance, in the judicial context, the concealed information test format might present less concern than the comparison question format, even if they have comparable accuracy. The concealed information test inquires about knowledge that is presumed to be possessed by the perpetrator; however, a “failed” test might only indicate that the subject lied about having been at the scene of the crime, not necessarily that he or she committed the crime. Like a fingerprint found on the murder weapon, knowledge of the scene and, possibly, the circumstances of the crime, is at least one inferential step away from the conclusion that the subject committed the crime. There may be an innocent explanation for the subject’s knowledge, just as there might be for the unfortunately deposited fingerprint. In contrast, the comparison question test requires no intervening inferences if the examiner’s opinion is accepted about whether the examinee was deceptive when asked about the pivotal issue. With this test, such an expert opinion would go directly to the credibility of the examinee and thus his or her culpability for the event in question. This possibility raises still another concern for courts, the possibility that the expert will invade the province of the fact finder. As a general rule, courts do not permit witnesses, expert or otherwise, to comment on the credibility of another person’s testimony (Mueller and Kirkpatrick, 1995). This is the jury’s (and sometimes the judge’s) job. As a practical matter, however, witnesses, and especially experts, regularly comment on the probable veracity of other witnesses, though almost never directly. The line between saying that a witness cannot be believed and that what the witness has said is not believable, is not a bright line. Courts, in practice, regularly permit experts to tread on credibility matters, especially psychological experts in such areas as repressed memories, post-traumatic stress disorder, and syndromes ranging from the battered woman syndrome to rape trauma syndrome. The legal meaning of a comparison question test polygraph report might be different if the expert opinion is presented in terms of whether the examinee showed “significant response” to relevant questions, rather than in terms of whether the responses “indicated deception.” Significant response is an inferential step away from any conclusion about credibility, in the sense that it is possible to offer innocent explanations of “significant response,” based on various psychological and physiological phenomena that might lead to a false positive test result. When courts assess the value of forensic tools, the consequences that follow a “positive” or “negative” outcome on the test are important. Al-
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The Polygraph and Lie Detection though scientific research can offer information regarding the error rates associated with the application of a test, it does not provide information on what amount of error is too much. This issue is a policy consideration that must be made on the basis of understanding the science well enough to appreciate the quantity of error, and judgment about the qualitative consequences of errors (the above discussion of errors and tradeoffs is thus relevant to considerations likely to face a court operating under the Daubert rule). Finally, evaluating the usefulness of a forensic tool requires a clear statement of the purpose behind the test’s use. With most forensic science procedures, the criterion is clear. The value of fingerprinting, handwriting identification, firearms identification, and bite marks is closely associated with their ability to accomplish the task of identification. This is a relatively straightforward assessment. Polygraph tests, however, have been advocated variously as lie detectors and as aids for interrogation. They might indeed be effective for one or the other, or even both. However, these hypotheses have to be separated for purposes of study. For purposes of science policy, policy makers should be clear about for which use they are approving—or disapproving—polygraphs. Courts have been decidedly more ambivalent toward polygraphs than the other branches of government. Courts do not need lie detectors, since juries already serve this function, a role that is constitutionally mandated. Policymakers in the executive and legislative branches, in contrast, do perceive a need for lie detection and may not care about whether the polygraph’s contribution is due to its scientific validity or to its value for interrogation. Mainstream Science and Forensic Science Many policy makers, lawyers, and judges have little training in science. Moreover, science is not a significant part of the law school curriculum and is not included on state bar exams. Criminal law classes, for the most part, do not cover forensic science or psychological syndromes, and torts classes do not discuss toxicology or epidemiology in analyzing toxic tort cases or product liability. Most law schools do not offer, much less require, basic classes on statistics or research methodology. In this respect, the law school curriculum has changed little in a century or more. The general acceptance test of admissibility enunciated in the Frye decision expects little scientific sophistication of lawyers or judges. Courts, and presumably juries as well, have thus evaluated expertise based on consensus. The problem with this test has come in fields that purport to be rigorous but may not be. For instance, if the question is the validity of bite mark identification analysis, researchers who study the
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The Polygraph and Lie Detection various factors that challenge this expertise (e.g., uniqueness of the mark, the identification of the mark in different substances, proficiency testing, etc.) would probably give the courts a solid scientific evaluation of the value of this kind of evidence. However, if the courts only consider the expert opinions of forensic odontologists who do bite mark identifications for police laboratories, they are unlikely to get a full view of the value of this kind of evidence. Unfortunately, in many fields of forensic science there are no communities of scientists conducting basic research and the only people who are asked as expert witnesses are interested practitioners with little proficiency in scientific methods. Good forensic science can have salutary results and, in some cases, profound consequences. DNA profiling is a particularly salient example of how good science can be used for both good law enforcement and in the interests of the falsely accused. Lawyers, under the influence of Daubert, are beginning to open their eyes and ears to empirical criticisms of fields long thought settled. In the area of lie detection, good forensic research could directly contribute to national security. Forensic science has not kept up with the state of science more generally for two basic reasons: the legal community’s basic ignorance of science and statistics, and the lack of interest among research scientists in the practical (and especially forensic) applications of science. In lie detection, for instance, policy makers have not demanded better work, and few scientists have been interested in pursuing the subject. This powerful combination of ignorance and apathy has, in general, deprived policy makers of good scientific data. More particularly, it has led to convictions of the innocent (see Scheck, Neufeld, and Dwyer, 2000), acquittals for the guilty, and numerous costs to individuals, ranging from job loss to social ostracism. Another institutional reality bears mentioning. The law very often asks empirical questions to which there are no scientific answers. Moreover, while science can take any amount of time to pursue a question and develop an answer, the law has to render a decision in a short time frame. A particularly good example of this is clinical prediction of violence. A large number of legal contexts call for predictions of future violence. These include capital sentencing, parole and pardon hearings, ordinary civil commitment, sexual predator commitments, and community notification laws. Courts and legislatures have been undeterred by the fact that psychologists and psychiatrists readily admit that science cannot provide such predictions—though the state of the art is improving. For policy makers, the inability to accomplish some task scientifically does not always mean that it cannot be done legally. In Schall v. Martin (1984), for instance, the Supreme Court upheld the pretrial detention of juveniles on a finding that there is a “serious risk” that if released the juvenile would
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The Polygraph and Lie Detection commit a crime before his next court appearance. Responding to the argument that such predictions could not be made reliably, the Court said that “our cases indicate, however, that from a legal point of view there is nothing inherently unattainable about a prediction of future criminal conduct” (at 278). CONCLUSIONS Decisions about whether or how to use polygraph tests in particular applications must consider these tests’ capabilities and limitations, as well as the tradeoffs posed by any imperfect diagnostic procedure. Tradeoffs in Interpretation The tradeoffs of false positives and false negatives are strikingly different in event-specific and screening applications, primarily because of the great difference in the base rate of guilt in the two settings. Even those who believe the polygraph “works” adequately in a criminal investigation should not presume without further careful analysis that this justifies its use for security screening. Given the very low base rates of major security violations, such as espionage, that almost certainly exist in settings such as the national weapons laboratories, as well as the scientifically plausible accuracy level of polygraph testing, polygraph screening is likely to identify at least hundreds of innocent employees as guilty for each spy or other major security threat correctly identified. The innocent will be indistinguishable from the guilty by polygraph alone. Consequently, policy makers face this choice: either the decision threshold must be set at such a level that there will be a low probability of catching a spy (thereby reducing the number of innocent examinees falsely identified), or investigative resources will have to be expended to investigate hundreds of cases in order to find whether there is indeed one guilty individual (or more) in a pool of many individuals who have positive polygraph results. Although there are reasons of utility that might be put forward to justify an agency’s use of a polygraph screening policy that produces a very low rate of positive results, such a policy will not identify most of the major security violators. In our judgment, the accuracy of polygraph testing for distinguishing actual or potential security violators from innocent test takers is insufficient to justify reliance on its use in employee security screening in federal agencies. Although formal benefit-cost analysis might in principle be used to help decision makers evaluate the difficult tradeoffs posed by the use of the polygraph for security screening, in actuality the scientific basis for
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The Polygraph and Lie Detection estimating many of the important parameters required for a benefit-cost analysis is too weak to support quantitative estimates. Moreover, no scientific basis exists for comparing on a single numerical scale many of the qualitatively different kinds of costs and of benefits that must be considered. The tradeoffs presented by polygraph testing vary with the application. For example, some focused screening applications may present more favorable tradeoffs for polygraph use than those involved in employee security screening in the DOE laboratories. Increasing Polygraph Effectiveness The quality control program organized by DoDPI and implemented by DOE in its screening activities is impressive in its rigor and the extent to which it has removed various sources of examiner and other variability. Highly reliable polygraph scoring and interpretation, such as these programs aim to provide, are essential if polygraph screening is to have scientific standing. Reliability, however, is insufficient to establish the validity of the polygraph for screening purposes. The effects of DoDPI efforts to increase reliability on the validity of polygraph screening are untested and unknown. The primary advances in polygraph technology since the 1983 Office of Technology Assessment report have come in the computerization of physiological responses and their display. Computerized polygraph scoring procedures have the potential in theory to increase the accuracy of polygraph testing because they improve the ability to extract and appropriately combine information from features of psychophysiological responses, both obvious and subtle, that may have differing diagnostic values. However, existing computerized polygraph scoring methods have a purely empirical base and are not backed by validated theory that would justify use of particular measures or features of the polygraph data. Such theory simply does not yet exist. Moreover, existing computerized polygraph scoring methods have not been tested on a sufficient number and variety of examinees after development to generate confidence that their validity is any greater than that of traditional scoring methods. Although in theory, combining the results of polygraph tests with information from other sources is possible—for example, in serial screening protocols—such approaches have not been seriously investigated. Similarily, evidence on the incremental validity of the polygraph, that is, its ability to add predictive value to what can be achieved by other methods, has not been gathered. Moreover, the difficulties that exist with computerized scoring of polygraph tests also exist, and may be multi-
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The Polygraph and Lie Detection plied, with possible expert systems for combining polygraph results with other data. Polygraphs in Legal Contexts Courts following the Daubert rule on admissibility of scientific evidence are likely to look increasingly to scientific validation studies in judging the uses of polygraph data in court. The existing validation studies have serious limitations. Laboratory test findings on polygraph validity are not a good guide to accuracy in field settings. They are likely to overestimate accuracy in field practice, but by an unknown amount. The available field studies are also likely to overestimate the accuracy achieved in actual practice. Assessments of the polygraph for the purposes of forensic science should take into account the test’s design, function, and purpose because both the accuracy of the test and the practical meaning of particular accuracy levels are likely to depend on these factors.
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The Polygraph and Lie Detection NOTES 1. This is the model we used to extrapolate A from reports that provided single sensitivity-specificity combinations (see Appendix H). 2. If A = 0.80, the false positive index is greater than 100 for any base rate below 1 in 250, and if A = 0.70, it is greater than 100 for any base rate below about 1 in 160. If the actual base rate is equal to or less than 1 in 1,000, the false positive index is at least 208 if the test has A = 0.90; at least 452 if A = 0.80; at least 634 if A = 0.70, and at least 741 if A = 0.60. Thus, if there are 10 serious security violators among 10,000 employees who are polygraphed and the criterion is set to correctly identify 8 of the 10, the test could be expected to erroneously classify as deceptive at least 1,664, 3,616, 5,072, or 5,928 of the 9,990 nonviolators, depending on which of the accuracy indexes applied to the test. 3. Other assumptions about the accuracy and sensitivity of polygraph testing procedures yield similarly dramatic differences between the predictive values of positive test results in screening versus event-specific investigation contexts. 4. A polygraph screening policy that produces 3 percent positive results, of which virtually all are false positives, will have a sensitivity of 48 percent (that is, it will correctly identify 48 percent of major violators) if the test procedure’s actual accuracy index (A) is 0.90; 25 percent if its accuracy index is 0.80; or 14 percent if its accuracy index is 0.70. 5. A polygraph screening policy that produces 1 percent positive results, of which virtually all are false positives, will have a sensitivity of 30 percent (identify 30 percent of the major violators) if the test procedure’s actual accuracy index (A) is 0.90; 13 percent if its accuracy index is 0.80; and 7 percent if its accuracy index is 0.70. 6. Polygraph testing of suspected Al Qaeda members is different from security screening of federal employees in other ways that should be recognized explicitly. Problems of language translation and of possible cultural differences in the meanings of deception and truthfulness are likely to create uncertainty in the meaning of polygraph charts and raise questions about whether these tests can be as accurate as similar tests conducted on English-speaking Americans. 7. We note that this criterion was rarely met in the simulation studies that have been used to assess polygraph validity for screening to date. 8. See United States v. Plaza, 188 F. Supp.2d 549, 2000 WL 389163 [E.D.Pa. March 13, 2002] vacating United States v. Plaza, 179 F. Supp.2d 492, 2002 WL 27305 [E.D.Pa Jan. 7, 2002]. 9. The implications of Daubert for polygraph evidence are not straightforward. Some courts have interpreted Daubert to undermine the per se rule excluding polygraph evidence (e.g., United States v. Posado, 57 F.3d 428, 429 [5th Cir. 1995]), and some federal district courts have admitted polygraph evidence. It is reasonable to expect continued argument in the courts over whether or not the scientific evidence on polygraph testing justifies the use of test results as evidence.
Representative terms from entire chapter: