the penile plethysmograph (PPG)

The penile plethysmograph 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 of such things as children being molested. Computer software is used to develop graphs showing "the degree of arousal to each stimulus." The machine costs about $8,000 and was 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. Behavioral Technology Inc. in Salt Lake City also sells the PPG machine. 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 say blood pressure measurement. [STATE OF NORTH CAROLINA v. ROBERT EARL SPENCER]

This much everyone seems to agree on: the device measures penile engorgment. Any male who has awakened with an erection knows, however, that penile engorgement is not always a measure of sexual arousal. On the other hand, most males would probably acknowledge that penile engorgement occurring while watching pornographic movies is due to sexual arousal. Why quibble? Let's grant that "when the penis becomes engorged, we are measuring sexual arousal." What utility could such a device possibly have?

Two uses have already been mentioned: to weed out false gays and in the treatment of sex-offenders. The latter is sometimes done in conjunction with aversion therapy, which involves "subjecting patients to electric shocks or putrid odors while they stare at sexually suggestive pictures. The idea is to dull the patient's interest in such materials. And in such cases, psychologists use the PPG to see how well the therapy is working." ["Monitoring device to treat sex offenders sparks controversy" August 6, 1996]

Submission to a PPG (as the test is called) 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. 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. And, researchers at the University of Georgia have tested the claim that homophobic men are latent homosexuals by using the PPG. In their study of 64 exclusively heterosexual men (self-identified) "sixty-six percent of the nonhomophobic group showed no significant erection while watching the male homosexual video, but only 20 percent of the homophobic men showed little or no evidence of arousal." ["Study Looks At Homophobic Men"]

Finally,

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. I once set up a lab in a psychiatry dept. at U. of Penn. to do this. [Dave Bunnell, personal correspondence]

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, studies done on the PPG have found that "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." (e.g. Freund, et al, 1972, Behavior Therapy, #6) And, in one court case, Dr. 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 upon 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 does 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 seem clear that evidence based on the PPG has no business in the courtroom, either to exculpate or incriminate.

Nevertheless, there is a whole 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.

One glaring problem with the use of the PPG is the lack of standardized materials to use as stimuli for subjects, a factor which 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 arousals 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 upon 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 into the armed forces, or for granting 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. 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 not a crime nor a sin 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. 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 presense 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. And 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 measurements validly indicate a tendency to commit or not commit sex crimes.

Reader comments

I am a psychologist in NY. Graduate of SUNY Buffalo. I've become more skeptical over the years.

I've received some heavy handed treatment of a certain powerful bureaucrats who insist that retarded individuals should be hooked up to penile plethysmographs to see if they're pedophiles. . . It's unbelievable! When you question their assumptions about this, they come back with veiled threats and ad hominem arguments.

The power of these people is scary.

Keep up the good work. . .

6 Nov 1996

Your argument holds true for the unauthorized use of the device in a prison or child custody situations, but in other cases it is a revolutionary and standardized method of determining the etiology of one's criminal persuasion.

reply: Revolutionary, perhaps, but in no way a standardized method of determining the etiology of anything, except perhaps the gullibility of soldiers, cops, prosecutors, parole boards, psychologists and social workers.

I think your "skeptical" cry to ban the device is little more than a witch hunt to eradicate its use.

reply: Thank you for noticing my "cry." Most people miss it. In any case, I haven't called for a "ban" on anything. You are partly right, though; I would like to eradicate the abuse of the PPG and see less wishful thinking and self-delusion in the world.

Perhaps addressing the inappropriate reasons for using the test should be focused on rather than the actual nuts-and-bolts of the setup.

Jim Roche, Seattle, WA

reply: I don't see why one can't do both.

Mr. Roche's follow up:
11 Nov 1996

You missed my whole point, charlie. Let me break it down to you. You popped up after an Infoseek search for the word "plethysmograph." I work in a residential care unit for witnessed sex offenders who for one reason or another stand incompetent to stand trial. They each received this test (albeit perhaps by an overzealous practitioner) to assess their compatibility with a community living environment. To make it simple, if they proved stimulated by a snapshot of a naked infant, their placement would be as far from infants as was possible. This expands to protect the public from any pursuits, because their conduct shows that they have low impulse control, and respond vigorously to stimuli that they see as appealing. ok? ok. I'm sure you're glad that there is a way, despite all of the financial, psychological, and bureaucratic hangups, to keep the predators away from your family and friends. Right? Thanks, jimr.

reply: "Residential care unit?" Is that the euphemism we now use for psychiatric facilities for sex-offenders? What kind of facility would consider placing "witnessed sex offenders who are incompetent to stand trial" anywhere in any community, except the community of other "witnessed sex offenders" and their caretakers? If your facility does, then I think the community needs to be protected from your facility as well.