The penile plethysmograph (PPG) is a controversial type of plethysmograph that measures changes in blood flow in the penis in response to audio and/or visual stimuli. 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.
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, Czechoslovakia, Hong Kong, New Zealand, Norway, Spain, and the United States.
There are two types of penile plethysmographs:
- 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.
Development and uses
The original air device was developed by researcher Kurt Freund in Czechoslovakia during the 1950s. The motivation for its development was to prevent draft dodgers from claiming that they were homosexual in order to avoid military service[How to reference and link to summary or text].
In the past, explicit or pornographic material was used as the stimuli set, but some modern systems use non-pornographic suggestive sets. This shifts the focus to the general arousal patterns rather than a specific arousal point[How to reference and link to summary or text].
The penile plethysmograph has value in screening organic versus psychogenic erectile dysfunction[How to reference and link to summary or text]. Lack of sexual response during REM sleep may indicate that further evaluation by a urologist is required[How to reference and link to summary or text].
The device has also been used in law enforcement and in psychological experiments. One study demonstrated that experimental subjects could be conditioned to show sexual arousal (as indicated by the device) in response to images of shoes, by initially preceding pictures of nude women with images of the shoes[How to reference and link to summary or text]. The experimenters inferred that the behaviourist concept of classical conditioning was likely to be an important influence in sexual fetishes.
For many years 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 doctrine. By 1993, after decades of use, the doctrine was rejected by United States Supreme Court 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. Some critics felt the Daubert decision lowered the standard of admissible scientific findings, while others felt that it answered some long-standing problems with Frye.
According to Barker and Howell, penile plethysmography (PPG) cannot meet most legal thresholds as a valid or relevant diagnostic tool 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 
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."
The penile plethysmograph is not intended to be used as a guilt or innocence tool, but rather as a supplemental device to add to a complete psychosexual evaluation. Some of the criticisms of PPG articulated by various criminal courts relate to the blatant misuse of PPG as a tool to determine whether a defendant committed a particular crime.
In recent years, a substantial amount of research data has been gathered and reviewed, and significant steps have been taken toward standardization.
The device has also been used in many states when evaluating convicted sex offenders. 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 Ninth Circuit Court of Appeals recently addressed the procedures required before a supervised release program could include penile plethysmograph testing. 
Courts in Canada came to a similar conclusion as the United States. While the Daubert decision was greeted with harsh criticism from those who feared a flood of "junk science" in courtrooms, the Supreme Court of Canada adopted the Daubert doctrine in R. v. J. (J.L.)  2 S.C.R. 600 with very little fanfare. Interestingly, this statement by the court was contained in a decision 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.
After developing the device in Czechoslovakia, developer Kurt Freund fled to Canada in the wake of the Prague Spring. Freund then began using the device at the Clarke Institute in Toronto, where much of the research and published data using the device originated.
Use of PPG followed in the wake of the "fruit machine," a slang term for another device developed in Canada in the 1950s and 1960s as part of a Royal Canadian Mounted Police campaign involving the identification and dismissal of every gay person in the employ of the public service.
- Skeptic's Dictionary on penile plethysmography
- Evidence of Penile Plethysmography, Psychological Profiles, Inventories and Other "Not a Pedophile" Character and Opinion Evidence Offered on Behalf of a Defendant in a Child Sexual Abuse Case is Inadmissible Under Daubert v. Merrell Dow Pharmaceutical
- "Can we identify the sexual predator by use of penile plethysmography?"
- The Penile Plethysmograph in False Allegation Cases
- Plethysmograph: A disputed device