Penile plethysmograph

The penile plethysmograph (PPG) is a controversial type of plethysmograph that measures changes in blood flow in the penis. Cavernous nerve penile plethysmograph measures changes in response to inter-operative electric stimulation during surgery.

For sexual offenders, it is typically used to determine the level of sexual arousal as the subject is exposed to sexually suggestive content, such as photos, movies or audio. It has been demonstrated by most studies to be the most accurate method of identifying which sexual offenders will go on to commit other sexual crimes against children, although clinicians have noted that this does not mean the test is appropriate for the evaluation of sexual preferences or treatment effects.

For prostatectomy nerve-sparing surgery, the surgeon applies a mild electrical stimulation near the Cavernous nerves of penis to verify their locations and avoid operative trauma. Damage to the difficult to see nerves can cause erectile dysfunction outcomes. At the surgery's conclusion, the electrical stimulation penile plethysmograph result is a prognostic which helps to manage the erectile function outcomes earlier than the many months required for recovery.


There are two types of penile plethysmograph:

  • Volumetric air chamber

When this is placed over the subject's penis, as tumescence increases, the air displaced is measured.

  • 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 to measure changes in circumference.

The circumferential type is more common, but the volumetric method is more accurate.

Significant suppliers of PPG machines include Behavioral Technology Inc. and Medical Monitoring Systems. The device is known to be used in Brazil, Britain, Canada, China, Czech Republic, Hong Kong, New Zealand, Norway, Slovak Republic, Spain, and the United States.

The surgical machine is supplied as CaverMap by Blue Torch Medical Technology, Inc.

A roughly equivalent procedure for women, vaginal photoplethysmography, measures blood through the walls of the vagina, which researchers claim increases during sexual arousal.


The original air device was developed by researcher Kurt Freund in Czechoslovakia during the 1950s. He later wrote, "In the early fifties homosexual interaction was still an indictable offense in Czechoslovakia. I was of course opposed to this measure, but I still thought, as did my colleagues at the psychiatric university hospital in Prague where I was working, that homosexuality was an experientially acquired neurosis" (p. 223) He then developed phallometry to replace psychoanalytic methods of assessment because "[P]sychoanalysis had turned out to be a failure, virtually unusable as an instrument for individual diagnosis or research....When phallometry began to look promising as a test of erotic sex and age preferences, we started using it mainly as a test of pedophilia, that is determining who has an erotic preference for children over adults" (p. 223-224).

In post-World-War-II Czechoslovakia, Freund was assigned by the communist government the task of identifying among military conscripts men who were falsely declaring themselves to be gay. "Freund (1957) developed the first device, which measured penile volume changes... to distinguish heterosexual and homosexual males for the Czechoslovakian army." When he escaped Europe for Canada, Freund was able to pursue his research using phallometry for the assessment of sexual offenders. At that time, attempts to develop methods of changing homosexual men into heterosexual men were being made by many sexologists, including John Bancroft, Albert Ellis, and William Masters of the Masters and Johnson Institute. Because phallometry showed that such methods were failures, Freund was among the first sexologists to declare that such attempts were unethical.

Reliability and validity

The Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) of the American Psychiatric Association states:

Erectile dysfunction

The penile plethysmograph has value in screening organic versus psychogenic erectile dysfunction. Lack of sexual response during REM sleep may indicate that further evaluation by a urologist is required.

When applied during nerve-sparing surgery, electrical stimulation penile plethysmograph is an erectile dysfunction prognostic. The patient is provided with objective information on their specific outcome which aids in planning for further erectile function therapies.

Sexual orientation

Phallometry has been shown to distinguish gay men from straight men.


It has been shown to distinguish paedophilic men from nonpaedophilic men.

Bradford's penile tumescence testing on 100 admitted child molesters correctly classified 62% of the admitted homosexual child molesters and 52% of the admitted heterosexual child molesters he tested.

In a study by Wormith, 42% of the paedophiles were classified as having normal sexual preferences.


Phallometry can distinguish men with erotic interests in cross-dressing from non-cross-dressers.


There is some evidence that phallometry can distinguish groups of men with a paraphilia involving rape from groups of men without it.

Legal admissibility

United States

Use as trial evidence

In the United States, a scientific technique could not be used as evidence in court unless the technique was "generally accepted" as reliable in the relevant scientific community. This was known as the Frye standard, adopted in 1923. In 1993, the doctrine was rejected by the Supreme Court of the United States in favor of a more comprehensive "reliable foundation" test in Daubert v. Merrell Dow Pharmaceuticals. In the Daubert standard, the "generally accepted" test was no longer determinative. Several other factors could now be considered, including whether the technique had been published and peer reviewed. Myers notes, "Courts that have considered penile plethysmography generally rule that the technique is not sufficiently reliable for use in court."

In United States v. Powers the court excluded the penile plethysmograph test because it failed to qualify under Daubert's scientific validity prong for two reasons: the scientific literature does not regard the test as a valid diagnostic tool, and "a vast majority of incest offenders who do not admit their guilt, such as Powers, show a normal reaction to the test. The Government argues that such false negatives render the test unreliable."

According to Barker and Howell, penile plethysmography (PPG) does not meet the legal threshold for the guilt phase for the following reasons:

  • No standardization
  • Test results are not sufficiently accurate
  • Results are subject to faking and voluntary control by test subjects
  • High incidence of false negatives and false positives
  • Results are open to interpretation

They concluded, "Until a way can be devised to detect and/or control false negatives and false positives, the validity of the test data will be questionable." Responding to Barker and Howell, Simon and Schouten noted, "Our own analysis suggests that the standardization and faking issues, as well as other problems not addressed in the Barker and Howell paper, warrant much more guarded conclusions about the use of the plethysmograph in legal and clinical settings." Prentky noted "the increased likelihood in forensic settings that dissimulation may compromise the validity of the assessment." Hall and Crowther noted penile plethysmography "may be even more problematic than other [methods] in assessing susceptibility of the test to faking."

In State of North Carolina v. Spencer, the 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."

More recently, a substantial amount of research data has been gathered and reviewed, and significant steps have been taken toward standardization. According to Launay (1999), "[T]he validity of the technique for research and clinical assessment is now established; it is only the use in guilt-determination proceedings that is inappropriate. Fedoroff and Moran called it an "experimental procedure" and noted, "Virtually every expert who has written about phallometry has cautioned that it is insufficiently sensitive or specific to be used to determine the guilt or innocence of a person accused of a sex crime."

Post-conviction use

Phallometry is widely considered appropriate for treatment and supervision of convicted sex offenders: "Courts have permitted plethysmographic testing for monitoring compliance by convicted sex offenders with the conditions of their community placement as part of crime-related treatment for sexual deviancy. Its use for the treatment and management of sexual offenders is recommended by the Association for the Treatment of Sexual Abusers. Becker notes it "should never be used exclusively in forensic decision making." The sexual assault trial of basketball player Kobe Bryant in Colorado brought this device and its use to public attention before the case was dropped in 2004, because Colorado law would have required evaluation with this device following conviction. The United States Court of Appeals for the Ninth Circuit recently addressed the procedures required before a supervised release program could include penile plethysmograph testing. The device is routinely used at civil commitment facilities, but "some clinicians and offenders say it is easy, particularly in a laboratory, to stifle arousal and thus cheat on a plethysmograph test." This has been reported to occur in 16% of cases.

During the Catholic sex abuse cases, the reliability of the test was questioned by some officials in the Roman Catholic Archdiocese of Philadelphia. Later, these officials chose to seek therapy at an institution where the plethysmograph was not used. This, even though the officials were made aware of the fact that the test was used by most experts and was believed to be of value in diagnosing sexual disorders. Later, a Grand Jury found that the Archdiocese of Philadelphia's decision to do so "had the effect of diminishing the validity of the evaluations and the likelihood that a priest would be diagnosed as a pedophile or ephebophile.


Courts in Canada came to a similar conclusion as the United States. The Supreme Court of Canada adopted the Daubert doctrine in R. v . J.-L.J. [2000] 2 S.C.R. 600, which upheld a lower court's decision to exclude testimony by a psychiatrist who had administered several tests on the accused, including a penile plethysmograph:


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