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The False Promise of the Penis Lie Detector Test

Originally touted as a barometer of men’s homosexuality or capacity to reoffend as a pedophile, the controversial device has been used everywhere from Army recruitment to conversion therapy to psych wards. Problem is, it’s probably penile pseudoscience

Editor’s Note: A previous version of this article stated that researchers Kurt Freund and Michael Seto had never worked together. However, the two published several book chapters and articles together in the 1990s. The article has now been updated to reflect this.

In police stations and forensic labs across the world, men are being subjected to what’s become colloquially known as a “penis lie detector” test. After measuring their own dicks, these men are usually invited into a solitary room, where they’re asked to attach a device to their penis that claims to measure who or what they’re sexually attracted to. The test’s subjects are typically sex offenders, and the test will supposedly prove it.

Joseph Plaud, a clinical and forensic psychologist based in Florida, has been administering these tests — called penile plethysmographs (PPGs) — for over 30 years. The phallometric assessment procedure is used to measure a person’s sexual arousal by monitoring the amount of blood in their erection in response to certain visual and auditory stimuli. The person being subjected to the procedure is hooked up to a monitor, and has a rubber band-type gauge filled with mercury attached to their penis. As they’re shown certain images or sounds — for example, naked people of varying ages — the gauges expand if the penis engorges with blood (monitored by the mercury sending electrical impulses to the PPG device). Supposedly, this determines which images stimulated them.

In 2007, the PPG was featured in the reality series, Rock of Love, with Poison’s lead singer Bret Michaels (for the uninitiated, it was a Bachelor-esque show in which female contestants rated Michaels’ test results to win a date with him). Then, in 2009, it made an appearance in Louis Theroux’s documentary, A Place for Paedophiles, which takes viewers inside Coalinga State Hospital in California, where sex offenders who have completed their prison sentences spend their subsequent years. Today, the PPG is typically used to monitor sex offender’s sexual preferences — primarily to establish if they’re pedophiles — but it can also be employed as a treatment for erectile dysfunction. In the past, it was also used to conduct gay aversion therapy, and has even been wielded on Army hopefuls to determine their sexuality.

It’s because of the latter usage that the procedure even exists. In the early 1950s, Kurt Freund, a Czech-Canadian physician and sexologist, was treating men for homosexual tendencies by administering aversion therapy, a psychological treatment in which a person is subjected to discomfort (like nausea-inducing drugs) while exposed to a particular stimulus (e.g., an erotic scene between two men). Because of his work in this field, Freund was commissioned by Czechoslovakian army commanders in 1953 to create a device that would identify anyone who was attempting to evade military service by pretending to be gay — those not turned on by erotic photos of men during Freund’s PPG were enlisted in the army. However, by 1957, Freund concluded that his early experiment was a failure. He determined that men couldn’t change their sexual preferences, and started advocating for the decriminalization of homosexuality, which was achieved in Czechoslovakia in 1961. He later apologized in 1977. 

As he moved away from “treating” homosexuality, Freund focused his PPG studies on the detection and diagnosis of sex offenders. “One benefit of the plethysmograph is that it can produce an objective measure of sexual arousal to children, which is beneficial for both risk assessment and treatment planning,” says Michael Seto, a psychiatry professor at the University of Ottawa and the editor-in-chief of the Sexual Abuse journal. Seto worked at Freund’s lab — the Clarke Institute of Psychiatry in Toronto, publishing several book chapters and articles with Freund, before Freund took his own life in 1996 after a battle with lung cancer. “Multiple studies show that [the PPG] is an important predictor of the likelihood men who have sexually offended will reoffend,” Seto tells me.

Plaud says the procedure is used to monitor how a sex offender is responding to treatment, and, based on the results, can influence how the individual’s program is altered. Talking me through how a PPG is administered, Plaud is quick to assert that “if done right,” the test doesn’t hurt, and at no point during it should the administrator see the subject’s genitals. Typically, he uses a model to instruct the subject how to fit the rubber gauges — they’re a little smaller than the circumference of the penis, which the subject will have already measured himself — before inviting them to sit down alone “in a comfortable chair,” where only their head and shoulders can be seen. 

The subject is then exposed to photos, videos or tapes, which Plaud shows through “virtual reality-type goggles” and headphones. “It doesn’t shock the person,” says Plaud, addressing the logistical ethics of the procedure. “It’s not uncomfortable. The gauge should be new and sterile, and we don’t see or touch anything.”

Yet despite Seto’s claim that it delivers objective results, and Plaud’s assurance that the procedure isn’t intrusive, the penile plethysmograph — like the polygraph before it — has faced its fair share of criticisms and controversies. As well as the accusation that a PPG is inherently flawed in its measurement of something as subjective and complex as arousal, critics claim that the device constantly churns out inaccurate results. This is, in part, because of its susceptibility to manipulation. “An examinee may prevent themselves from showing arousal by distracting themselves or engaging in other techniques to restrict arousal,” says Emily Gottfried, an associate professor of psychiatry and behavioral sciences at the Medical University of South Carolina.

Answering questions in a Reddit AMA eight years ago, one sex offender — who was on probation at the time — discussed how he attempted to cheat the test. He recalled being shown a series of photos of children of various ages in their underwear — one age/gender at a time. After each set of images, a male voice read out a scenario that might be “coercive, violent, maybe he was spying from a distance (voyeurism) and jerking off while watching someone,” and he had to “hit the button whenever we heard something inappropriate.” At the end of the test, the subject was shown “a series of adult, uncensored female nudes.” “When I took the test, figuring I was supposed to be turned on by that,” he wrote, “I tried my best to get a chubby going during that part. However, I was told that a problem — that I showed objectification issues for showing arousal to that section. You could never win with these people.”

Plaud says he attempts to avoid manipulation by flashing a red, green or blue dot at random intervals, and asking the subject to note when they see it. He’s also previously asked subjects to describe what they just watched. However, Gottfried admits that because of the possibility of deceit, “using the PPG in absence of any other data would be a failing of its use.”

Furthermore, both Plaud and Gottfried make clear the distinction between being aroused by non-consensual or abusive stimuli and actually acting on that arousal. “It’s not illegal to be a pedophile,” says Plaud. “It’s illegal to act on it. You have to be crystal-clear that someone shouldn’t be damned even if the evaluation turns out to be 100 percent accurate — that doesn’t mean they did it.” Similarly, a 2015 study found that 62 percent of women admit to having rape fantasies, but — of course — that doesn’t mean they actually want to experience sexual assault. As declared by Psychology Today at the time, “in fantasy, everything is permitted and nothing is wrong. As sexual openness increases, so does willingness to daydream about sexual scenarios one would never really want to experience.”

Karl Hanson, a psychologist who co-authored two studies on penile plethysmography and sexual recidivism, says PPG assessments can often be “unnecessary when deviant sexual interests can be assessed by self-report or offense history.” He adds: “It’s only in a minority of cases where these assessments provide information that could not have otherwise been gleaned from the case file.” The test can also come back inconclusive, whether via manipulation or because of the sterile setting of the procedure. 

In a 2016 interview with VICE, one sex offender — whose results were inconclusive — recalls being forced into undergoing a PPG in 1991, despite telling the police he didn’t want to do it. “My attorney said I didn’t have a choice,” Ron (a pseudonym) explained. “He said if I didn’t, the judge would give me a max sentence anyway. It seemed very invasive and demeaning.”

Plaud asserts this would never happen today. “Debriefing and explanation are very important, and it should always be the case that an individual has informed consent and can withdraw their consent at any time,” he says. “If they start participating and change their minds, [the PPG] is terminated immediately.” Plaud does admit, however, that the procedure can be distressing for those undergoing it, and it can be “uncomfortable from a psychological perspective.” 

But the biggest ethical issue, according to Seto, is the use of real child pornography in some administrating laboratories. “In the past, the child stimuli involved real children [with parents’ consent], which raises additional issues,” says Hanson. “In Canada, the possession and distribution of obscene materials is permitted under federal law for legitimate medical or research purposes. This isn’t the case in many U.S. states. Consequently, PPG evaluators who want to tailor their materials to specific interests [e.g., bestiality] risk criminal prosecution.” However, Hanson adds that today “almost all PPG stimuli are computer-generated avatars, not real people.”

Despite its numerous, vast-ranging ethical issues, it seems the penile plethysmograph is here to stay. Plaud says developments in technology over the years since its invention have made the procedure a lot easier to administer — particularly because the device is now pocket-sized — and have “hopefully allowed for a lot more reliability.” Plaud also points to another assessment, called the Abel Assessment for Sexual Interest (created by Gene Abel in 1995), which was designed as a replacement for the PPG. However, the technique has been declared inadmissible in court in various jurisdictions, and its methodology (which involves measuring how long a subject looks at an image to determine their sexual interest in it) has been widely criticized. 

Even so, experts continue to support the use of these kinds of assessments, in spite of questions about their reliability, accuracy and morality. Ultimately, while the jury’s still out on their impact on reoffending, Plaud confirms that the penile plethysmograph is useless in a court of law. “It can be used, and it can be misused,” he concludes. “It’s only as good as the person administering it, and if they’re not trained, or don’t have the right experience or equipment, then it’s going to be problematic — and could end in big trouble. Unless they admit to it, you’re on very shaky ground.”