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Penile Plethysmograph (PPG)

a/k/a ‘The Peter-Meter’

 “Plethysmography is a term applied to the measurement of volume and volume displacement.” The penile plethysmograph (PPG) measures changes in blood flow in 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 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, and to keep homosexuals out of the military. 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 (used in the United States ).

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.

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.

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 been used in law enforcement and in psychological experiments. In the US , PPG testing is administered at hundreds of hospitals, correctional institutions, and treatment facilities for sex offenders. It is commonplace in China , Brazil , New Zealand , Canada , and the UK . In Brussels, the Centre de Recherche-Action en Sexo-Criminologie has used PPG testing since the early 90s.

Dr. Barbara Schwartz, a respected clinical psychologist, stresses that PPG results are useful only when they show "clear, repeated patterns of arousal" that are corroborated by other means. Most state agencies managing sex offenders claim to comply with guidelines formulated by the Association for the Treatment of Sex Abuse (ATSA), a professional organization for clinicians and researchers. ATSA plethysmography standards ("Initial physiological assessments should only be interpreted in conjunction with a comprehensive psychological examination," etc.) are intended to insure that the test is properly conducted, and that its results are interpreted in the light of collateral data.

Significant suppliers of PPG machines include Farrall Instruments Inc., of Grand Island, Nebraska, Parks Medical Electronics, Inc., Las Vegas, Medical Monitoring Systems of New Jersey and Behavioral Technology Inc. in Salt Lake City.