Thanks! It looks pretty unreliable, and its results are not necessarily meaningful.
penile plethysmograph (PPG)
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 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. 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.
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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. 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 of 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. Psychologists use the PPG to measure the success of the therapy.
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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."
More:
http://skepdic.com/penilep.html