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The Penile Plethysmograph in False Allegation Cases

Prior to our discussion, please understand that there are basically two types of testing used in the treatment and the identification of a possible sex offender. The penile plethysmograph, or PPG, and the Abel Assessment, a test developed by Gene G. Abel, M.D. Prior to developing the Abel Assessment, Dr. Abel personally used the plethysmograph, but believing that in many cases it could be very degrading for the person being tested, he developed the Abel Assessment to accomplish the same results as the plethysmograph, but without the embarrassment or the use of nudity or pornography. The penile plethysmograph measures penile circumference in millimeters caused by a partial or total erection. It is a good, direct measure of physiological arousal because erection rarely occurs except to sexual stimulation.

The Abel Screen, on the other hand, uses an entirely different method. Both the plethysmograph and the Abel Assessment are very good methods of measuring a sexual preference, such as pedophilia and both tests pass all the requirements for scientific measure. The catch is that there are a few people who molest children who are not sexually aroused by children, such as psychopaths, alcoholics and child porn persons who are in it for the money, but they can usually be detected with psychological tests. Although not perfect, the accuracy of both of these devices are probably the best of any psychological tools.

Unfortunately, one of the most controversial subjects imaginable is the use of the Abel Assessment or penile plethysmograph and their use, especially in preparing a defense for anyone falsely accused of sexual abuse. All testing of this nature is admitted depending on the laws governing the state where you reside. It is an absolute disgrace when anyone who has been falsely accused has to "prove" their innocence, but in most cases, they are required to do just that. These tests, administered and used properly, can greatly assist in that area.

As previously stated, the penile plethysmograph is a machine that measures 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 plethysmograph directly measures the outside evidence of sexual arousal. When a man becomes sexually aroused, there is engorgement of the penis. When the penis becomes engorged, you are measuring sexual arousal, so for all practical purposes, the test is a blood pressure measurement.

The tests has been used to weed out false gays, in the treatment of sex-offenders, in sentencing decisions for sex offenders, as a condition of parole for certain sex offenders and in some child custody cases to determine if a father is or is not likely to abuse his child. Some psychologists use the PPG to measure the success of the therapy.

Some studies done on the PPG, however, 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. 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." 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."

The reliability 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."

There is a substantial difference of opinion within the scientific community regarding the plethysmograph's reliability to measure sexual deviancy. See e.g., Barker and Howell, The Plethysmograph: A Review of Recent Literature, 20 Bull. Am. Acad. of Psychiatry and Law 13 (1992) (identifying several problems with the reliability of the plethysmograph, namely "lack of standards for training and interpretation of data, lack of norms and standardization and susceptibility of the data to false negatives and false positives," and concluding that "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").

Most courts applying the Daubert analysis have excluded evidence of penile plethysmography based upon its scientific unreliability. In United States v. Powers, 59 F.3d 1460 (4th Cir. 1995), the court excluded the penile plethysmograph test because it failed to qualify under Daubert's scientific validity prong. The evidence produced at trial clearly showed that these factors weighed against the admission of the penile plethysmograph test results. First, the Government proffered evidence that the scientific literature addressing penile plethysmography does not regard the test as a valid diagnostic tool because, although useful for treatment of sex offenders, it has no accepted standards in the scientific community. Second, the Government also introduced evidence before the judge that a vast majority of incest offenders who do not admit their guilt show a normal reaction to the test. The Government argues that such false negatives render the test unreliable. The Powers court noted the fact that it was unable to locate any decisions acknowledging the validity of penile plethysmography other than in the treatment and monitoring of sex offenders. See also, Garren v. State, 1996 WL 37987, 220 Ga. App 66, 467 S.E.2d 365 (1996)(sustaining trial court's refusal to admit evidence of test).

Most, if not all, of the courts that have reached the issue of the admissibility of penile plethysmography tests have excluded evidence of the tests. See e.g., See, e.g. R.D. v. State, 706 So. 2d 770(Ala. Crim. App. 1997); Nelson v. Jones, 781 P.2d 964 (Alaska 1989) cert. denied, 498 U.S. 810 (1990)(judge sitting as fact finder in family relations matter rightfully gave no weight to penile plethysmograph evidence as unreliable); People v. Stoll, 49 Cal. 3d 1136, 265 Cal. Rptr. 111, 783 P.2d 698, 713 n. 21 (1989) (dicta); People v. John W., 185 Cal. App. 3d 801, 229 Cal. Rptr. 783, 785 (1st DCA 1986) (defendant failed to establish that the plethysmograph "was a reliable means of diagnosing sexual deviance" and ruled the test inadmissible). See also In re Mark C. v. San Diego County Dept. of Social Services v. David C. , 7 Cal. App. 4th 433, 445,8 Cal. Rptr. 2d 856 (1992)(penile plethysmography and other test battery excluded); Gentry v. State, 213 Ga. App. 24, 443 S.E.2d 667 (1994); Stowers v. State, 215 Ga. App. 338, 449 S.E.2d 690 (1994); Dutchess Cty. Dept. of Social Services v. Mr. G., 141 Misc.2d 641, 534 N.Y.S.2d 64, 71 (1988) ("the results of the plethysmograph as a predicator of human behavior cannot be considered"); Cooke v. Naylor, 573 A.2d 376 (Me. 1990); State v. Ambrosia, 67 Ohio App. 3d 552, 587 N.E.2d 892, 899 (1990)(penile plethysmography unreliable in classifying pedophilia); In the Interest of A.V., 849 S.W.2d 393 (Tex. Ct. App. 1993) (proponent of test failed to establish its reliability).

Penile plethysmography cannot meet Daubert's validity or relevance tests because the test results are not generally accepted, are not sufficiently accurate, the test is subject to faking and voluntary control by test subjects, there is no standardized set of stimuli or scoring used by plethysmography experts and the results are not relevant to the question of whether the Defendant is a child molester.

No test can determine whether a person will act on his feelings and desires. Anyone planning on taking the test should be aware that studies have shown:

  • Plethysmograph data is totally useless for determining guilt or innocence regarding deviant sexual acts. It would be like using a personality test to convict someone of burglary.

  • Plethysmographic data have no diagnostic value of any kind.

  • About one-third of offenders show no arousal in the lab.

  • The proper stimuli to use are not pictures, but taped fantasies. Ideally taped fantasies spoken by the offender themselves, describing the offenders' own fantasies as well as contrived ones.

  • The value in plethysmographic assessment is to aid the offender in learning how to control erection responses during deviant fantasy. That is it. It has no value in "seeing if the treatment worked." Treatment should not be court-ordered.

While the majority of the above has been about the plethysmograph, the Abel Assessment has also taken a number of hits from the overall scientific comminity. In many cases, when one talks to individuals who use the plethysmograph, many are quick to "trash" the Abel Screen, however, the fact is, both work. This argument is akin to a polygraph examiner "trashing" a "voice stress" test. Both work, but the polygraph was here first, so, according to the polygraph examiner, it must be the best.

The Abel Assessment has been used more than 200,000 times in the United States. It has passed all Daubert Standards in western Louisiana and in State Courts in Texas and a number of other states, but the same arguments that apply to the plethysmogrpah would also apply to the Abel Assessment. Both are used and both are widely accepted in the overall scientific community as valid testing for "treatment" but not for "identifying" a sexual offender. That is the catch, especially when attempting to admit the "testing" as a defense to a false allegation cases. There is, however, a method that I have found to be successful in my cases. Normally, a plethysmogrpah or Abel Screen alone are simply tests and, I know those tests will probably not pass the Daubert Standards. I have found a great deal of success in having my clients complete a two-day battery of tests, conducted by Dr. Abel personally. This battery consists of psychological testing, the Abel Assessment, a polygraph, a drug screen and in-depth interviews by Dr. Abel. This is far more than a simple Abel Assessment alone and, as a result, we have success in having Dr. Abel personally admitted as a defense expert. I would expect the same would be true with the plethysmograph, as long as it was not the primary reason for admitting the expert.