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Plethysmograph: a disputed device

A genital plethysmograph (pronounced pluh-THIZ-muh-graf) is a controversial device that measures blood flow in the genitals. Sensing equipment is attached to the subject’s genitalia and the subject’s reactions are measured while exposed to images or audio recordings of materials some people find erotic.

Proponents claim the devices are an accurate gauge of sexual arousal, although results are highly variable and open to interpretation and manipulation by both the test subject and the observer.

Most work with plethysmographs has been done using penile plethymography (PPG), but some researchers have recently attempted to create vaginal plethysmography. Researchers note that like its penile counterpart, vaginal probing has many disadvantages, including “the lack of a sound theoretical basis for interpreting what exactly the signal means” (Meston 2000).

Some researchers and clinicians in the field of sexual offender treatment accept penile plethysmography as a useful tool in assessing and treating admitted offenders and in monitoring offenders post-incarceration. However, even among those clinicians who accept and use plethysmography, the majority do not feel that plethysmography has any usefulness as a diagnostic tool or as a reliable predictor.

Plethysmography and gender variance

The study of penis measurements is called phallometrics, and Kurt Freund is considered a primary influence in that field of study. Freund developed the device in the 1950’s to catch recruits in Czechoslovakia trying to get out of the military by claiming to be gay. Freund later immigrated to Canada and was the head of the gender program at Toronto’s notorious Clarke Institute, where he continued his work on penile plethysmography.

Ray Blanchard replaced Freund at the Clarke Institute in 1995.

Since Freund’s suicide in 1996, his legacy has continued to have a damaging effect on gender-diverse people. Freund and Blanchard used their sex offender assessment methodologies to assess gender-diverse children and adults, under the assumption that gender diversity is a type of mental disorder.

The only places where this device is still used on gender-diverse people are countries where Freund worked. A transgender activist in the Czech Republic wrote in 2000:

This machine is used in diagnostics of transsexuals (both Females to Males and Males to Females) here in the Czech republic. Some of the doctors ask the clients to undergo the test with PPG as the obligatory test for making a diagnosis of transsexuality! They show erotic photos to transsexuals. Of course, we transsexuals consider this to be total nonsense. They think it is science… We consider this test to be very humiliating and at the same time absolutely useless in these cases. (Carroll 2003)

Most mental health professionals do not consider this device a useful or reliable diagnostic tool, but the Clarke Institute and Ray Blanchard continue to assert that the device allows them to diagnose and predict the life trajectories of gender-diverse clients. Blanchard used it to help formulate his opinions that gender diversity is a psychosexual pathology in those not attracted to males (a disease he made up called “autogynephilia“).

Quack attack

As with many quacks, some of Blanchard’s work is legitimate science. Blanchard has made several important scientific contributions regarding birth order and sexual orientation (Blanchard 1996), and has done some interesting anthropometric work looking for patterns among other demographic groups.

However, his greatest error has been the overemphasis on interpretation of subjective data from PPG. My work in the field of consumer fraud has trained me to spot overpromotion in the field of health, and Ray Blanchard’s work with PPG is textbook quackery.

I am a proud affiliate of QuackWatch, whose founder Stephen Barrett, M.D. describes how quacks like Blanchard operate:

Quackery is not an all-or-nothing phenomenon. A practitioner may be scientific in many respects and only minimally involved in unscientific practices. To avoid semantic problems, quackery could be broadly defined as “anything involving overpromotion in the field of health.” This definition would include questionable ideas as well as questionable products and services, regardless of the sincerity of their promoters. (Barrett 2001)

Blanchard’s plethysmograph promotion has remarkable similarities to many of the quack hair removal devices I have helped put out of business:

  • Heavily promoted by the people who invented or popularized it.
  • Usefulness or efficacy is drastically overstated.
  • Subjective results are presented as objective data.

There are two types of penile plethysmographs in use:

Volumetric air chamber

  • This is the type invented by Freund and still in use at the Clarke Institute. When this is placed over the subject’s penis, as tumescence increases, the air displaced is measured. Disadvantages of the device: it’s “expensive, cumbersome, tricky to calibrate, little freedom for vertical growth.” (Winters 2004)

Circumferential transducer

  • This uses a mercury-in-rubber or indium/gallium-in-rubber ring strain gauge and is placed around the shaft of the subject’s penis. Though most research has been done with this device it is considered less accurate.

A vaginal photoplethysmograph is more complicated, because it measures the amount of blood in the genitalia by monitoring minor changes in skin color inside the vagina. It is essentially similar to a lie detector that measures blush response.

Canada’s “fruit machine” quack device

The plethysmograph wasn’t the only device Canadians were using to identify “homosexual” and “nonhomosexual” citizens when Freund arrived from Czechoslovakia. Since almost two decades earlier, Royal Canadian Mounted Police (RCMP) were developing a “fruit machine,” a slang term for a failed program to develop a device which could detect sexual orientation by measuring the involuntary responses of subjects exposed to erotic pictures.

Subjects were placed in a dental office style chair with a pulley attached to a camera going towards the pupils. A black box in front showed pictures of men and women that ranged from non-sexual to sexually explicit. Subjects were made to view pornography, and the device measured the pupils of the eyes, perspiration, and pulse for a supposed erotic response. Subjects were told the machine measured stress.

The “fruit machine” was employed in Canada in the 1950s and 1960s during a campaign to eliminate all homosexuals from civil service, the RCMP, and the military. In 1981, the Royal Commission received a “massive report on RCMP wrongdoings” by Justice David McDonald after four years of research. The report confirmed “what was already known and widely feared. The RCMP’s Security Service has committed numerous illegal acts, ranging from mail- tampering to kidnapping and arson. It has lied to responsible government ministers. It has carried out surveillance of Canadian citizens on a far grander scale than anyone suspected.” (Loos 1981) RCMP had files on 800,000 citizens — one out of every 30 Canadians. 

The McDonald Report confirmed surveillance of all known gays in Ottawa. The effort included a plan to record the movement of all known Ottawa gays on a gigantic map of the city. More bizarre and sinister are the activities, beginning in the 1950s, of RCMP Security Service subsection A-3, whose sole purpose “was the identification and dismissal of every gay person in the employ of the public service” (Sawatsky 1980).

After knowledge of its real purpose became widespread, few people volunteered for it. A substantial number of workers did lose their jobs. Although funding for the “fruit machine” project was cut off in the late 1960s, the investigations continued, and the RCMP collected files on over 9,000 suspected gay men and lesbians. The McDonald Report recommended “that the existing Security Service files on homosexuals be reviewed and that those which do not fall within the guidelines for opening and maintaining files be destroyed.”

Clearly, devices like these are not scientific and prone to massive abuse. That’s why the “fruit machine” and plethysmograph are not allowed as evidence in court cases.

Scientifically unreliable in the eyes of the law

In the United States, a scientific technique can’t be used as evidence in court unless the technique is “generally accepted” as reliable in the relevant scientific community. This was decided by the Supreme Court in 1993 and is called the Daubert Standard (Daubert 1993).

Plethysmography cannot meet most legal thresholds as a valid or relevant diagnostic tool for the following reasons:

No standardization

One widely quoted criticism of plethysmography is the lack of standardized test stimuli and lack of a determination regarding which of the many aspects of stimuli cause arousal and how that data correlates to behavior (e.g. Simon 1993, pp. 506-07).

There is inadequate description of stimuli in studies, categories of stimuli are imprecise because people exhibit idiosyncratic and overlapping patterns of preference and arousal. (Howes 1995).

Susan Smith notes in her excellent overview of the literature:

In the deposition of Bradford (1997), at 129, he states that here is no uniform set of visual or audio stimuli used by plesythmography testers. There is also no uniform scoring procedure and no agreement on what degree of arousal is clinically significant and what conclusions can be inferred from percentages of engorgement. (Simon 1991, p.. 78-79)

No standard set of stimuli or scoring is used by plethysmography experts, and standardized test results are not generally accepted.

Test results are not sufficiently accurate

Bradford (1997) candidly reviews the problems in penile response in testing. He collected data from a group of 200 subjects, 100 of whom were admitted child molesters and 100 of which were volunteer “community control” subjects. Even using an admitted population of child molesters, Dr. Bradford’s penile tumescence testing could only correctly classify 62% of the admitted homosexual child molesters and 52% of the admitted heterosexual child molesters. According to the report, twenty-five (25%) accuracy in classification would have been expected from mere chance. 

Other studies report that plethysmography has an even poorer accuracy record. Simon & Schouten cite a study (Wormith) in which 42% of the pedophiles were classified as having normal sexual preferences. Another study they cite (Barbaree and Marshall) found that only 35% of pedophiles demonstrated the “pure” child-preference profile (Simon 1993, at 508). See also McAnulty 1990.

The primary concern, according to Bradford, “is the external validity of the procedure, or the extent to which the assessment of sexual preference in the laboratory predicts behaviour outside the laboratory” (Bradford 1997).

Results are subject to faking and voluntary control by test subjects

The largest and most unresolved problem with using measurements of penile response as a diagnostic or classification tool is the fact that penile response can be controlled by test subjects. “The problem of faking, more than any other, continues to limit the clinical utility of phallometric tests” (Schouten 1992).

In one test, researchers made the “striking and somewhat unexpected finding [of] the ability among 80% of the subjects who exhibited sexual arousal to the stimuli to later voluntarily and completely inhibit sexual arousal.” (Hall 1988) Of Hall’s 122-member test group, 114 were able to reduce their response, 91 were able to completely inhibit response and only 4 were unable to reduce their arousal. Problems of accuracy in classification increase when the test is used with a non-admitting population because response is subject to voluntary control and the test is therefore vulnerable to faked response.

In another study, it was found that all the participants were able to suppress their responses to stimuli in order to give misleading data as to their sexual preference (Kaine 1988).

In a recent study, it was confirmed that the majority of test subjects could fake enhanced phallometric outcomes for age preference (Wilson 1998).

One legal review notes the problem with the reliability of penile plethysmograph testing is that penile response is subject to voluntary control, and the test should not be used to determine whether or not an individual has engaged in deviant behavior (Myers 1989). No reliable means exist for reducing the impact of faking on test results (Simon 1991).

False negatives and false positives

Barker & Howell (1992) discuss PPG’s lack of usefulness for determining guilt or innocence or for predicting future offenses. 

For instance, according to Bradford (1997), only 40% of child molesters are classified as pedophiles (p. 213). Johnston (1997) notes one-third of the molesters in study identified as fixated and 2/3 as regressed.; Abel (1988) states that the majority of subjects in the study could become involved with adult partners; paraphilic and nonparaphilic behavior and coexisted in most subjects.

Therefore, even giving the plethysmograph the benefit of the doubt on scientific reliability, the question whether someone is aroused by stimulli has little correlation and is not probative on the issue of whether they will act on those feelings.

In a lab setting, the accuracy of penile plethysmography on subjects who are involved in evaluation is even more problematic. Studies have shown that there is a heightened potential for an increased number of false-positive and false-negative results, as well as incidents of faking in offenders involved in adjudicatory process.

Results are open to interpretation

The problem with plethysmographs is that they are like lie detectors (polygraphs): they measure a body response, but the data is open to interpretation. For this reason, they are often challenged as evidence in court, as with lie detectors.

This is the major controversy in Blanchard’s work: interpretation of data, and issues of his subjectivity, based on his assumption that transition is about erotic preference. While this may describe someone like Blanchard acolyte Anne Lawrence, who considers sex drive “that which moves us most,” many of us feel this is not an accurate or even correct description of our motivation for transition. Cause and effect may be difficult to distinguish.

Even Blanchard’s mentor concluded in one study that plethysmography not dependable for diagnosis due to the ability of many subjects to influence the outcome. (Freund 1989). The question of determining via plethysmography whether or not a person will behave in a predictable way “is not one that clinical assessment can address” (Becker 1993). “The value of this procedure may be most notably compromised in the realm of interpretation” (Howes 2003).

Unreliable for predicting behavior

As the Skeptic’s Dictionary notes:

A man or woman may be aroused by the sight of animals copulating or be aroused by a film of a woman eating a banana and a man eating a fig in particularly provocative ways. Still, they may have no desire to engage in bestiality or have sex with a bowl of fruit. A heterosexual man or woman may be aroused by the sight of lesbians engaging in oral sex, but have no desire to have sex with lesbians or in the presence of lesbians.

Strong arousal need not imply strong desire for what causes the arousal; and weak arousal need not imply weak desire. Furthermore, no test can determine whether a person will act on his feelings and desires.

One expert quipped that using plethysmograph results for evidence “would be like using a personality test to convict someone of burglary” (Stein 1997).

In State v. Spencer (1995), the North Carolina court reviewed the literature and case law and concluded that penile plethysmography was scientifically unreliable: “Despite the sophistication of the current equipment technology, a question remains whether the information emitted is a valid and reliable means of assessing sexual preference” (Barker 1992).

Scientifically unreliable in the eyes of mental health professionals

The doubt expressed by legal and scientific experts in the citations above echoes the same concerns by professional trade groups involved in psychology and mental health.

The Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) of the American Psychiatric Association (“DSM-IV”), universally accepted by courts and mental health professionals as authoritative in the area of the diagnosis of mental disorders and diseases, states:

Penile plethysmography has been used in research settings to assess various paraphilias by measuring an individual’s sexual arousal in response to visual and auditory stimuli. The reliability and validity of this procedure in clinical assessment have not been well established, and clinical experience suggests that subjects can simulate response by manipulating mental images. DSM-IV, Paraphilias, at 524.

Because the DSM-IV is the diagnostic “bible” for mental health professionals engaged in assessment of mental illness and disorders, its rejection of penile plethysmography as an assessment tool signifies “non-acceptance” of the methodology within the scientific community.

Experts speak out on plethysmograph abuses

Robert M. Stein, Ph.D. from the Center for Neurobehavioral Health, Ltd. Lancaster, PA comments on Carroll 2003:

From 1982-1988, I was the Director of the Psychophysiological lab at the Sexual Behavior Clinic in New York City. I personally (not through a technician) assessed and treated over 700 adolescent and adult sex offenders. After reading your piece on the plethysmograph, I have a few comments: 

1. Plethysmograph data is totally useless for determining guilt or innocence regarding deviant sexual acts. It would be like using a personality test to convict someone of burglary. 

2. Plethysmographic data have no diagnostic value of any kind. 

3. About one-third of offenders show no arousal in the lab. 

4. The proper stimuli to use are not pictures, but taped fantasies. Ideally taped fantasies spoken by the offender themselves, describing the offenders’ own fantasies as well as contrived ones. 

5. The value in plethysmographic assessment is to aid the offender in learning how to control erection responses during deviant fantasy. that is it. It has no value in “seeing if the treatment worked.” Treatment should not be court-ordered. The best outcomes I’ve seen were post-adjudication, when there was no coercion involving treatment. I had the occasion to do some treatment with post-adjudication offenders at a detention facility in Bucks County, PA. 

6. The plethymograph most definitely has value in diagnosing organic versus psychogenic impotence. A study by Dr. Charles Fisher in 1965 determined that 1,000 out of 1,000 healthy men tested had penile erection responses during REM sleep. When there is lack of response, that indicates further evaluation by a urologist. So it is useful as a screening device.

In summary, I agree with many of your concerns regarding the plethysmograph. I may have more direct, hands-on experience with this device than anyone in this country, and I’ve spoken against its abuses for some time now. (Stein 1997)

Although studies suggest that arousal and sexual activity do not necessarily go hand in hand, that’s not going to stop people like Bailey, Blanchard, and Lawrence, who see that their plethysmograph is about to go the way of the doornail in favor of brain scans. They need funding and they need it now. Other researchers are beating them at their own game, so they seem to be jockeying plethysmograph-based arousal results from last year (Chivers 2003) as some sort of brain scan data. Most legitimate scientists are moving away from this outdated and error-prone device to functional magnetic resonance imaging, or fMRI, and other brain scanning procedures. Unfortunately, Blanchard and his protege Bailey have an aspiration to fame which has them seeking the limelight (Dotinga 2004) and doing science by press release (Tremmel 2003).

The real long-term issue with Bailey and Blanchard is “thought crime” described by Orwell in 1984 or “future crime” described by Philip K. Dick in “The Minority Report.” Their claims about the predictive value of neurometrics sound a lot like phrenology revisited.

Bailey, Blanchard, and Lawrence embrace a model in which gender variance is associated with sex offenses, criminality, mental illness, and lying. Blanchard asks in one paper “what defect in a male’s capacity for sexual learning” could cause gender variance (Blanchard 1991). This parole officer mentality permeates their theories and methods, which are carried out under the assumption that those who agree with them are “honest and open” (Bailey 2003, 147) or “unusually candid” (Lawrence 1998). Those who disagree are lying.

When challenged by transsexual women, they frequently try to discredit critics by accusing them of deception. In his widely denounced book, Bailey quotes a Blanchard employee who asserts that “most gender patients lie” or “provide misleading information” (Bailey 2003, 172-175). Bailey also attempts to dismiss scientific criticism as “political correctness” (Pinnel 2003) or “identity politics” that are “a hindrance to scientific truth” (Bailey 2003a). He claims that “there is good scientific evidence that you should believe me and not them” (Dreier 2003).

The level of hostility they show towards gender-variant people and peers who take issue with their methods puts the author in mind of the study (Adams 1996) that linked homophobia to homosexual arousal and is reminiscent of the abuses of McCarthyism.

Bailey, Blanchard, Lawrence, and plethysmograph abuse

No test can determine whether a person will act on feelings and desires. There is no controlled scientific data demonstrating that a person who gets aroused by certain imagery or sounds is significantly more likely to seek feminizing procedures than one who does not get aroused. On the other hand, there is no compelling evidence that a person who does not get aroused by certain imagery or sounds is significantly less likely to to seek feminizing procedures.

That hasn’t stopped Bailey and friends from making sweeping unsubstantiated claims about sexuality in gender-variant women based on plethysmographic guesswork (Latty 2003):

To rule out the possibility that the differences between men’s and women’s genital sexual arousal patterns might be due to the different ways that genital arousal is measured in men and women, the Northwestern researchers identified a subset of subjects: postoperative transsexuals who began life as men but had surgery to construct artificial vaginas.

In a sense, those transsexuals have the brains of men but the genitals of women. Their psychological and genital arousal patterns matched those of men — those who like men were more aroused by male stimuli and those who like women were more aroused by the female stimuli — even though their genital arousal was measured in the same way women’s was.

“This shows that the sex difference that we found is real and almost certainly due to a sex difference in the brain,” said Bailey. (Tremmel 2003)

The authors of “Men Trapped in Men’s Bodies” (Lawrence 1998) and The Man Who Would Be Queen (Bailey 2003) did not choose their titles just for provocation. They seek to prove that gender-variant women are really men, with “brains of men” (Tremmel 2003) who display “male-typical” sexual arousal (Lawrence 2003). They also want to use us to claim that sexual orientation is immutable by asserting gender-variant women who change their dating preferences after transition didn’t really change their orientation:

One transsexual participant who reported a change in sexual orientation following sex reassignment displayed genital and subjective responses consistent with her pre-reassignment sexual orientation (Lawrence 2003).

However, Lori Brotto and colleagues completely refute Lawrence’s assertions in a 2005 study which determined the “movement artefacts interfered with our assessment of the genital arousal response”:

In conclusion, the findings from this study suggest that the vaginal photoplethysmograph can- not be considered a viable technique to study the blood flow patterns in the neovagina during conditions of sexual arousal (Brotto 2005).

The most insulting and aggravating assertion made by Bailey, Blanchard, and Lawrence is that any gender-variant woman not exclusively attracted to males exhibits a paraphilia, and that in any relationship after transition, their partner is nothing more than a prop in a paraphilic script. While this may explain Anne Lawrence’s obsession with ritualized genital modification (Smith 1997) and inappropriate sexual behavior in professional settings (James 2003), Lawrence’s insistence that this behavior is typical or even diagnostically indicative of transsexualism seems less about science and more about an axe to grind with a community that frequently rejects Lawrence’s divisive and confrontational assertions.

Our community cannot stand by while we are treated like sex offenders (or more accurately, gender offenders) by a handful of pseudoscientists who must assert transsexual women are males in order to validate their theories and identities.

Until gender-variant children and adults are no longer subjected to outdated and humiliating plethysmographic procedures in order to get access to treatment, activists have considerable work to do. The potential for abuses with plethysmography will continue to remain a threat as long as people like Bailey, Blanchard, and Lawrence have any influence in their field.

We need to make sure these publicity-hungry hacks take a back seat to real neuroscientists and biologists, while they stick to “peter meters” and other anthropometric pursuits worthy of their talents.

Acknowledgement: This article is heavily indebted to the outstanding plethysmography research of Susan K. Smith, attorney.

Revision of 12 April 2006. Originally published 16 May 2004.

Citation: James AJ (2004). Plethysmograph: a disputed device. From Version of 12 April 2006.


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