The Penile Plethysmograph (PPG)

Two types of testing are often discussed in the evaluation and treatment of people accused or convicted of sex offenses: the penile plethysmograph, or PPG, and the Abel Assessment for Sexual Interest, a test developed by Gene G. Abel, M.D. These tests can come up in child sexual abuse cases, including when a person who has been falsely accused wants to show that they do not have a sexual interest in children. This article explains what the tests are, why their reliability is disputed, and how courts, including Texas courts, have treated them as evidence. If you have been accused or arrested, see our guide to what to do after an arrest in Texas for an overview of your rights and next steps.

What Is a Penile Plethysmograph?

The penile plethysmograph measures changes in the circumference of the penis as an indicator of physical sexual arousal. A stretchable gauge is placed around the penis and connected to recording equipment. While the person views sexual images or listens to audio descriptions, the equipment records changes, including changes the person may not notice, and software produces graphs showing the response to each stimulus.

Supporters describe the PPG as a direct measure of physical arousal. Critics point out that arousal in a laboratory is not the same as behavior, and that the results depend heavily on the stimuli used, how the test is administered, and how the results are interpreted.

The PPG has been used in the treatment of people convicted of sex offenses, in sentencing and supervision decisions, and in some custody disputes. Some clinicians also use it to measure progress in treatment.

What Is the Abel Assessment?

Dr. Abel developed the Abel Assessment as an alternative to the plethysmograph that does not require nudity, physical measurement, or explicit material. Instead of measuring physical arousal, it relies on how long a person views images of people of different ages and genders, a method often called viewing time. Like the PPG, it is used most often in the evaluation and treatment of people who have been convicted of sex offenses.

Why the Reliability of These Tests Is Disputed

There is a substantial difference of opinion in the scientific community about whether these tests can reliably identify sexual deviancy, and whether a “normal” result says anything about whether a person committed a particular act. Concerns that courts and researchers have identified include:

• A lack of standardized stimuli, scoring, and norms;
• The risk of both false positives and false negatives;
• The possibility that a test subject can consciously control or fake responses; and
• The fact that not everyone who commits an offense against a child is sexually attracted to children, so an arousal pattern cannot answer whether someone committed the act alleged.

A 1992 review of the research, cited by the North Carolina Court of Appeals in State v. Spencer, 119 N.C. App. 662, 459 S.E.2d 812 (1995), identified “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 concluded 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.” (Barker and Howell, The Plethysmograph: A Review of Recent Literature, 20 Bull. Am. Acad. of Psychiatry and Law 13 (1992).)

No test can determine whether a person will act on their feelings or whether a person committed a specific act.

How Courts Have Treated PPG Evidence

Courts that have considered PPG results offered to show that an accused person is not sexually interested in children have generally excluded them.

In State v. Spencer, a defendant accused of sexually abusing his five-year-old stepdaughter offered testimony from a psychologist, Dr. Gullick, that his plethysmograph results showed an essentially normal arousal pattern. The trial court allowed Dr. Gullick to testify only to opinions that were not based on the plethysmograph, finding:

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.

The Court of Appeals agreed.

In United States v. Powers, 59 F.3d 1460 (4th Cir. 1995), a federal appeals court upheld the exclusion of PPG results under the Daubert standard. The government presented evidence that the scientific literature did not regard the test as a valid diagnostic tool, although it was useful in treating sex offenders, and that most incest offenders who do not admit guilt show a normal reaction to the test.

Courts in other states have reached similar conclusions, including Nelson v. Jones, 781 P.2d 964 (Alaska 1989), Cooke v. Naylor, 573 A.2d 376 (Me. 1990), and Gentry v. State, 213 Ga. App. 24, 443 S.E.2d 667 (1994).

PPG and Abel Assessment Evidence in Texas

Texas criminal courts decide whether scientific expert testimony is admissible under Texas Rule of Evidence 702 and the reliability standard from Kelly v. State, 824 S.W.2d 568 (Tex. Crim. App. 1992). The party offering the evidence must show that the underlying theory and technique are valid and were properly applied.

Texas appellate courts have been skeptical of both tests:

• In In the Interest of A.V., 849 S.W.2d 393 (Tex. App.–Fort Worth 1993), a parental-rights case, the court expressed grave reservations about whether the penile plethysmograph had been shown to be reliable or generally accepted.
• In In re C.D.K., 64 S.W.3d 679 (Tex. App.–Amarillo 2002), the court held Abel Assessment testimony inadmissible because its reliability had not been established.
• In Figueroa v. State, No. 04-08-00452-CR (Tex. App.–San Antonio 2009) (memorandum opinion), the court upheld a trial court’s exclusion of Abel Assessment testimony offered by the defense in an indecency with a child case, while the defense expert was allowed to testify about a separate clinical evaluation.

What This Means for the Falsely Accused

It can feel deeply unfair that someone who has been falsely accused feels pressure to prove their innocence. In a criminal case, the burden is always on the State to prove guilt beyond a reasonable doubt. Still, many people who are falsely accused look for ways to show they do not fit the allegation, and testing is often suggested.

Before agreeing to a PPG, an Abel Assessment, or any similar test, talk with your defense attorney. A test result on its own is unlikely to be admitted to prove innocence, and the results could be used in ways you do not expect. In some cases, a broader psychological evaluation by a qualified expert, using methods that can meet the reliability standard, may be more useful than a single test, and the attorney can evaluate whether any evaluation should be done at all and how its results would be protected.

For related information, see our articles on false allegations, expert testimony issues in false allegation cases, and the falsely accused at trial.

Falsely accused of a sex crime in Texas? Call Dunham & Jones 24/7 at 800-499-8455 for a free consultation.

Frequently Asked Questions About the Penile Plethysmograph

Frequently asked questions about the penile plethysmograph and the Abel Assessment.

This article provides general information and is not legal advice. Every case depends on its particular facts, charges, criminal history, court orders, and applicable law.