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penile plethysmograph (PPG)

"A prisoner should not be compelled to stimulate himself sexually in order for the government to get a sense of his current proclivities." --U.S. 9th Circuit Court of Appeals Judge John Noonan

The penile plethysmograph (pluh-THIZ-muh-graf) (PPG) is a machine for measuring changes in the circumference of the penis. A stretchable band with mercury in it is fitted around the subject's penis. The band is connected to a machine with a video screen and data recorder. Any changes in penis size, even those not felt by the subject, are recorded while the subject views sexually suggestive or pornographic pictures, slides, or movies, or listens to audio tapes with descriptions of such things as children being molested. Computer software is used to develop graphs showing "the degree of arousal to each stimulus." The machine cost about $8,000 when first developed in Czechoslovakia to prevent draft dodgers from claiming they were gay just to avoid military duty. Farrall Instruments Inc., of Grand Island, Nebraska, manufactures the device and in 1993 had sold most of the 400 units then in use in sex-offender treatment centers in some 40 states. Medical Monitoring Systems of New Jersey is also one of the leading PPG manufacturers. Another vendor of the PPG is Behavioral Technology Inc. in Salt Lake City. In addition to the United States, the device is being used in China, Hong Kong, Norway, Britain, Brazil and Spain.

The theory behind the device is described by Dr. Eugenia Gullick

The plethysmograph . . . directly measures the outside evidence of sexual arousal. We know-- it's established throughout the literature that when a man becomes sexually aroused--there is engorgement of the penis. It's a one-to-one relationship.

In a polygraph, galvanic skin responses are measured, and we have to make a leap of logic to think that galvanic skin response is related to anxiety, and therefore truthfulness. And it is that jump in logic that leads to a lack of reliability at times with that instrument . . . .

We know when the penis becomes engorged, we are measuring sexual arousal. So it's much more akin to ... blood pressure measurement. (State of North Carolina v. Robert Earl Spencer, 1995)

This much everyone seems to agree on: the device measures penile engorgement. Any male who has awakened with an erection knows, however, that penile engorgement is not always a measure of sexual arousal or sexual desire. On the other hand, most males would probably acknowledge that penile engorgement occurring while watching pornographic movies is due to sexual arousal. 

What utility could such a device possibly have? Two uses have already been mentioned: to weed out false gays and to treat sex-offenders. The latter is sometimes done in conjunction with aversion therapy, which involves subjecting patients to electric shocks or foul odors while being shown sexually suggestive pictures. The hope is that the treatment will dull the patient's interest in sexy materials. The device can also be used in conjunction with cognitive behavioral therapy in which the patient allegedly learns how to control his responses to situations that might trigger sexual arousal or desire. Psychologists use the PPG to measure the success of the therapy.

Submission to a PPG has been made a condition of parole for certain sex offenders. The PPG has been used in child-custody cases to determine that a father is or is not likely to abuse his child, and in sentencing decisions for sex offenders. It has even been given to children as young as 10 who had abused other children. The latter was done in Phoenix, Arizona, with no evidence either that the test was useful or that it would not be harmful when given to children. Not everyone submits quietly to the PPG requirements, however. Officials in Old Town, Maine, had to pay nearly a million dollars to a policeman who was threatened with firing for refusing to submit to a PPG.

Despite the lack of a theoretical basis for interpreting the data gathered using the PPG, Professor Henry E. Adams et al. of the University of Georgia used the PPG to measure arousal of heterosexual men who were divided into homophobes and non-homophobes. They published their results in the Journal of Abnormal Psychology in 1996:

The results of this study indicate that individuals who score in the homophobic range and admit negative affect toward homosexuality demonstrate significant sexual arousal to male homosexual erotic stimuli.*

In their study of 64 exclusively heterosexual men (self-identified), 66 percent of the non-homophobic group showed no significant arousal while watching a male homosexual video, while only 20 percent of the homophobic men showed little or no evidence of arousal.

Adams notes that there are at least two competing explanations for the fact that homophobic men would be aroused by "male homosexual erotic stimuli." One is the Freudian explanation in terms of latent homosexuality. Despite their protests, these heterosexual homophobes are secret homosexuals. Another explanation, however, is that

viewing homosexual stimuli causes negative emotions such as anxiety in homophobic men but not in non-homophobic men. Because anxiety has been shown to enhance arousal and erection, this theory would predict increases in erection in homophobic men. Furthermore, it would indicate that a response to homosexual stimuli is a function of the threat condition rather than sexual arousal per se.

There may be other explanations, as well, but we have no way at present to determine which, if any, are valid.


there is an area where this device makes a valuable contribution: that of sorting out organic from psychogenic impotence. This is done by measuring changes in penile circumference during sleep, with increases expected during REM sleep. Men with psychogenic impotence still show erections, while those with an organic problem don't. It works. (Dave Bunnell, personal correspondence, who says he once set up a lab in the psychiatry department at the University of Pennsylvania to do this.)

Finally, there are some therapists who claim that chemical castration will significantly reduce or eliminate sexual arousal that leads to criminal behavior by sexual predators. However, clinical psychologist Jesus Padilla, who works with sexual predators at Atascadero State Hospital, "measured arousal rates of men who had been chemically castrated against men who had not by using a device that shows the men photographs of children and women in various nonsexual poses. He found no difference in arousal rates between Atascadero patients who had been castrated and those who had not" (Sacramento Bee, Special Report on Sexual Predators, February 14, 2006). While the Bee does not describe the device used in this test, it was likely the plethysmograph. In 1996, California implemented a law that keeps Sexually Violent Predators (SVPs) after they've served their sentences. SVPs are those who are "deemed likely to commit sexually violent acts in the future following completion of their prison sentences." A treatment program for SVPs was instituted and part of the treatment includes "a willingness to complete specific assessment procedures such as a plethysmograph evaluation."* [The state of California has not revealed much about this program, but Mareva Brown and Sam Stanton of the Sacramento Bee did a detailed investigative report on it. Part 1, Feb. 12, 2006; Part 2, Feb. 13; Part 3, Feb. 14.]

where's the science?

Scientifically, what are we to make of such a device? Well, the machine can measure response time to a stimulus and it can measure change in penile girth over time. Apparently, it is assumed that the more quickly aroused and the greater the engorgement the higher the "arousal level." Apparently, it is also assumed by many practitioners that any "arousal level" when viewing or listening to descriptions of naked children or adults having sex with children is "deviant." Yet, according to studies done by the inventor of the PPG, Dr. Kurt Freund, "many so called normal men who have not committed illegal sex acts show considerable arousal to stimuli depicting naked children or children involved in sexual activity."* And, in one court case (State of North Carolina v. Robert Earl Spencer, 1995), Dr. William Michael Tyson, a clinical and forensic psychologist specializing in the field of sexual criminal behavior, testified that "the vast majority of individuals who commit sexual offenses against children are not sexually aroused by stimulus material involving children." His expert adversary in that case, Dr. Gullick, claimed that "the plethysmograph has been extensively studied and recently shown to be ninety-five percent accurate in discriminating between individuals who had committed sexual offenses against children and a control group that was randomly drawn from the population." Yet, other experts have claimed that there are "studies in which the devices have failed to detect nearly one out of three known sex offenders tested."

It seems to be the case that the device has been the subject of many scientific studies and the results have been mixed, to put it kindly. The reliability and utility of the device have been argued in court and penile plethysmographic evidence has been declared inadmissible because of its "questionable reliability." The case in which Dr. Tyson testified was heard by the North Carolina Court of Appeals. That Appeals Court upheld a lower court's exclusion of testimony by Dr. Gullick because her testimony was based on the use of the penile plethysmograph. The defendant in the case was accused of sexually molesting his 5-year old stepdaughter. He called Dr. Gullick to testify that his "arousal pattern" when tested on the plethysmograph indicated that he was not aroused by children. Presumably, the defense believed that this was strong evidence that he didn't molest the child. The trial court ruled that "Dr. Gullick would be permitted to testify as to any opinions which were not based on the plethysmograph." The Appeals Court agreed with the trial court that "the instrument was of questionable reliability; that the testimony was not relevant; and that even if relevant, its probative value was outweighed by its prejudicial effect."

We agree with the trial court that the evidence before it by no means established the reliability of the plethysmograph; there is a substantial difference of opinion within the scientific community regarding the plethysmograph's reliability to measure sexual deviancy....

In the present case, plethysmograph testing formed the basis for Dr. Gullick's opinion that defendant was not sexually aroused by children, thereby making it less likely that he committed the acts charged. In view of the lack of general acceptance of the plethysmograph's validity and utility and therefore, its reliability for forensic purposes in the scientific community in which it is employed, we hold that the trial court did not abuse its discretion in finding defendant's plethysmograph testing data insufficiently reliable to provide a basis for the opinion testimony which defendant sought to elicit from Dr. Gullick. (State of North Carolina v. Robert Earl Spencer)

Dr. Tyson testified in the Spencer case that it was "generally accepted in the mental health community by both proponents and opponents of the plethysmograph that the plethysmograph data do not give any evidence that is useful in determining whether an individual did or did not commit a specific act. He also noted that "there is substantial disagreement as to the extent to which the penile response is subject to voluntary control and as to whether the penile response as measured by the plethysmograph can then be generalized to anything else pertaining to sexual behavior." Putting it mildly, Dr. Tyson claims that the plethysmograph has very limited forensic utility. It seems clear that evidence based on the PPG has no business in the courtroom, either to exculpate or incriminate.

Nevertheless, there is a growing industry of therapists who treat sex offenders and think the PPG will assist them "in determining whether someone who has committed a sex crime has a pattern of deviant sexual interests." Therapists use the PPG to help them devise treatment programs and to measure the success of their treatment. All this is done without any concern, apparently, that there is no compelling evidence that sexual arousal or non-arousal from pictures or sounds significantly correlates with criminal deviant behavior. There is no compelling evidence that a person who gets aroused by pictures or sounds is significantly more likely to commit sex crimes 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 pictures or sounds is significantly less likely to commit sex crimes than one who does get aroused.

Still, the PPG can provide some information which might prove useful to a sex-offender therapist. The computer software used with the PPG enables the tester to develop graphs that indicate whether the subject is more aroused by males than by females, by children than by adults, by coerced than by consensual sex, etc. The therapeutic controversy begins, however, as soon as the therapist tries to convert "arousal levels" to anything meaningful, such as claims that a sex-offender is "cured" or is "responding positively to treatment." This is in addition to the controversy already mentioned over using the PPG in conjunction with aversion therapy or cognitive/behavioral therapy.

One glaring problem with the use of the PPG is the lack of standardized materials to use as stimuli for subjects, a factor that clearly biases the data. Therapists vary greatly in the kind of materials they use to arouse subjects. Some materials are rather tame, e.g., nude adults, children in underwear or bathing suits. Others use hardcore pornography, including depictions of rape and pedophilia. Furthermore, there is no standard of "deviancy" for arousal. Worse, if therapists can define certain arousal as deviant, they can then suggest treatments for the deviancy as well as having the power to declare when the "deviant" is "cured." Convicted sex-offenders are in no position to protest either declarations that they have "deviant arousals" or treatments forced on them in the name of curing them of the "disease" of "deviant arousal."

More objectionable than the questionable scientific validity of the device, however, are the moral and legal questions its use raises. Some of the materials would probably be illegal on the open market because they constitute child pornography. Much of the material is morally objectionable. Some of the uses of the device raise constitutional issues. For example, submission to the PPG test as a condition for employment, for enlistment in the armed forces, or for being granted custody of children. Some penal institutions have made submission to the PPG a condition of parole, even though the device's usefulness as a predictor of behavior is unproven. The practice has been upheld by the Seventh Circuit Court of Appeals (1995). Parole Boards have great latitude in establishing conditions for parole. These conditions do not have to meet the same rigorous standards as are required before something allegedly scientific can be admitted as evidence in a trial. Nor do the normal liberties and constitutional protections of citizenship automatically apply to one being paroled.

From a scientific, moral, and legal point of view, what should matter is whether a person gives in to perverse desires and commits sex crimes. It is neither immoral nor a crime to get aroused. Furthermore, being aroused is not identical to having a desire. 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 or have sex at all. 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. And, if Dr. Tyson is correct, apparently there are many "normal" men who are aroused by photos of naked children but have no desire to have sex with children. There are many pedophiles who are not aroused by photos of naked children. The PPG measures arousal, not desire, though many sex-offender therapists seem to identify arousal with desire. These therapists, therefore, may be engaging in little more than wishful thinking when they think that because they witness a decrease in arousal they have evidence for a decrease in desire, which they correctly correlate with a decreased likelihood of acting on that desire. Decreased arousal may not be strong evidence for decreased tendency to engage in criminal sex acts. 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. Nevertheless, many of those who treat sex offenders swear by the PPG even though there is no compelling evidence that PPG readings validly indicate a tendency to commit or not commit sex crimes.

In case you are wondering, there is a similar device for measuring female arousal. G. Sintchak and J. H. Geer created a vaginal plethysmograph in 1975, but vaginal probing for signs of arousal is no more reliable than penile measurements. There is no sound theoretical basis for interpreting what the vaginal photoplethysmographmeasurements mean (Meston 2000). "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."*

reader comments

further reading


Throwing Away the Key: Is a lifetime in jail the best way to deal with sex offenders? Sex-offender treatment strategies generally fall into two categories, says [Pamela] Schultz, "behavioral treatments, which have to do with the physical impulses [and] those that focus on what's above the neck." In the first category, the aim is to "deaden the sexual impulse," either through "chemical castration" with hormonal drugs such as Depo-Provera or through exposure to unpleasant sensations—the smell of ammonia, for example—in conjunction with sexually suggestive images. (Counselors test whether this last treatment is working with a device called a penile plethysmograph, which measures blood flow and arousal.) Literal castration is sometimes used—it's popular in Texas—but as Slate has noted, "even castrated men are often still able to maintain an erection, and some castrated men have managed to reoffend."

Toronto: global epicenter for oppression of sex and gender minorities One device used at the Centre for Addiction and Mental Health (CAMH, formerly the Provincial Lunatic Asylum) is the penile plethysmograph. Though most courts treat this plethysmograph like a polygraph and deem it scientifically unreliable and inadmissible as evidence in criminal trials, that hasn't stopped the CAMH people from using it to create evidence about all kinds of sex and gender minorities.

Penile Plethysmography Testing for Convicted Sex Offenders Plethysmography - Testing Requirements for Supervised Release of Sex Offenders Deemed an Undue Deprivation of Liberty When Less Invasive Testing Methods Are Available - Journal of the American Academy of Psychiatry and the Law; 35:4:536-537 (2007).

Use of the Penile Plethysmograph in the Assessment and Treatment of Sex Offenders - Report of the Interagency Council on Sex Offender Treatment to the Senate Interim Committee on Health and Human Services and the Senate Committee on Criminal Justice January, 1993 Glen Kercber, Pb.D.

Adler, Stephen J. (1993). "Debatable Device," Wall Street Journal, February 3, p. 1.

Kokish, Ron, "Sex Offender Treatment Does it Work? Is it Worth it?"

Meston, C. M. (2000). The psychophysiological assessment of female sexual function. Journal of Sex Education and Therapy, 25(1), 6-16.

Sintchak, G., Geer, J. H. (1975). A vaginal photoplethysmograph system. Psychophysiology 12:113-115.


Attorney Susan K. Smith's arguments regarding admissibility of penile plethysmography under Daubert v. Merrell Dow Pharmaceutical

The Penile Plethysmograph in False Allegation Cases

Plethysmograph: a disputed device  by AJ James (2004).

court cases

Court limits test used on released sex offenders -  Judges say genital response measuring should be 'last resort' (US v Weber)

United States Court of Appeals For the Seventh Circuit: John E. Walrath v. Carol P. Getty and United States Parole Commission (1995)

U.S. Appeals Court Rejects Arousal Test for Sex Offender by Tanya Caldwell, LA Times

Parker & Parker vs. David Dodgion, Ph.D


 Once Ridiculed, Male Bisexuals Are for Real Researchers placed a penile plethysmograph -- a rubber band-shaped band -- on the base of men's penises while they watched videos of male to male and female to female sex. "We measured actual physical arousal," said Rosenthal. "You can't create arousal when it isn't there for them. You can't fake it."

(comment: True, but sexual arousal by something is not the same as a proclivity to, attraction to, or desire to engage in sex with that something.)

Czech "peter meter" test for gay asylum seekers slammed by EU "If you think a TSA grope is bad, gay men seeking asylum in the Czech Republic are forced to drop their pants and get hooked up to a penile plethysmograph to find out just how gay they are. After the "peter meter" device is attached to their penises, they are subjected to all kinds of pornography to see what happens to their junk. This test then determines if they can seek asylum."

Canadian 'peter meter' youth program halted; tester charged with sexual assault "...The final straw was when one of the test administrators was arrested for a sexual assault allegedly committed during leisure time....The current guy in charge of Sexual Abuse is, unsurprisingly, a CAMH employee, so he is a huge proponent of penile plethysmography. In fact, you can often find him on Wikipedia altering articles on sexuality to promote theories and devices his coworkers developed via the CAMH Phallometry Lab (an actual tax-funded Toronto lab)...."

B.C. suspends penile sex tests on young offenders "...the B.C. Civil Liberties Association demanded the government intervene after it learned of the tests. Within hours, the government suspended the sex testing after the provincial advocate for children and youth announced she would conduct a review....During the test, a youth would attach a device to his penis that is designed to measure his physical sexual arousal. Researchers in another room then play images of adults having sex, followed by images of naked children and infants, as they monitor the youth's level of arousal...."

Last updated 21-Oct-2015

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