Introduction
Penile plethysmography (PPG) measures how blood flow to the penis changes. It’s a testing method used mainly by forensic psychologists and clinicians. The test records physical changes when a person views different images or hears various sounds.
PPG has been used for over 50 years. It helps assess sexual arousal patterns, especially in legal settings and treatment programs. Though controversial, it remains an important tool in specific fields.
This article explains PPG testing, how it works, what it’s used for, and why some people question its use. Better understanding can lead to more informed talks about its role in psychology and treatment.
What is Penile Plethysmography?
PPG measures blood flow changes to the penis during arousal. When sexual arousal happens, blood flows to the penis, causing it to change in size. The test tracks these changes.
Czech psychiatrist Kurt Freund first developed this technique in the 1950s. He used it to study sexual orientation. Today, experts use it mainly to assess sexual interest patterns, especially in forensic cases.
Types of PPG Measurement
There are two main ways to measure with PPG:
- Volumetric Assessment: This method uses an airtight tube placed over the penis. As the penis grows larger, it displaces air in the tube. The test measures these air changes.
- Circumferential Assessment: This more common method uses a thin gauge placed around the penis shaft. The gauge stretches as the penis grows in size, measuring changes in its circumference.
Testing Procedure
A typical PPG test follows these steps:
- Preparation: The person learns about the test and gives consent (in non-criminal settings). Staff explain privacy measures and prepare the testing room.
- Setup: The person enters a private room. They attach the measuring device themselves, with no one else present.
- Baseline Measurement: Initial readings establish normal measurements when the person isn’t aroused.
- Stimulus Presentation: The person views or hears various materials, which may include:
- Pictures or videos
- Recorded stories describing different scenarios
- Written materials
- Response Recording: Throughout the test, equipment records changes in penis size. These changes compare to the baseline measurements.
- Data Analysis: After testing, experts review the results. They look for patterns of arousal to different types of stimuli.
The testing follows strict rules to ensure fair results. People can usually stop the test at any time.
Clinical and Forensic Applications
Sex Offender Assessment and Treatment
PPG is most often used with sex offenders. It helps clinicians:
- Find arousal patterns that people might not admit to
- Set starting points before treatment begins
- Track changes during and after treatment
- Assess risk of reoffending based on arousal patterns
Diagnostic Applications
Less commonly, PPG may help with:
- Telling the difference between physical and mental causes of erectile problems
- Assessing unusual sexual interests (though this use is debated)
- Research on sexual responses in different groups
Legal System Use
In some court systems, PPG testing may be:
- Required as part of sex offender evaluation
- Used in decisions about parole or probation
- Part of treatment plans for convicted sex offenders
- Used in evaluations for civil commitment
Scientific Validity and Reliability
The science behind PPG testing is complex:
Strengths
- Many studies show PPG can tell the difference between offenders and non-offenders
- When done right, PPG shows fairly good reliability
- Research suggests it can detect arousal patterns that people try to hide
A large study by Hanson and Bussière (1998) found that unusual sexual preferences measured by PPG strongly predicted reoffending.
Limitations
- Test results vary when repeated with the same person
- People can try to control their responses
- Different labs may get different results
- Some men don’t respond enough for valid measurement
- Some people with unusual interests show no physical response during testing
A major review by Marshall and Fernandez (2000) found PPG has value. But its scientific foundation has gaps that limit how results should be used.
Ethical Considerations and Criticisms
PPG testing raises several important ethical questions:
Privacy and Dignity Concerns
- The test is invasive and can be embarrassing
- Maintaining dignity during testing is challenging
- Privacy protections are critically important
Consent Issues
- In legal settings, testing may feel forced if tied to parole
- True informed consent may be hard to get in these cases
- People should know exactly how their data will be used
Legal and Rights Concerns
Courts in some areas have questioned PPG testing:
- The 9th Circuit Court in the US has raised concerns about required PPG testing
- Some question whether it violates rights against unreasonable search
- Many courts limit how PPG evidence can be used
Stimulus Material Ethical Issues
The materials used in PPG testing raise questions:
- Creating, having, or using certain materials may be illegal
- Many testing centers now use computer-generated images
- Viewing problematic material might reinforce harmful interests
Alternatives and Complementary Approaches
PPG isn’t the only way to assess sexual interests:
Viewing Time Measures
- Tracks how long people look at different images
- Less invasive than PPG but still objective
- The Abel Assessment is one example
Self-Report Measures
- Uses surveys and interviews about sexual interests
- Less physically intrusive but people can lie
- Includes tools like the Multiphasic Sex Inventory
Implicit Association Tests
- Measures automatic mental connections
- Can reveal unconscious attitudes about sex
- Harder to fake than self-reports
Eye-Tracking Technology
- Records where eyes look when viewing images
- Shows what draws visual attention
- Less invasive than PPG
Many experts recommend using several methods together rather than PPG alone.
Current Status in Clinical Practice
PPG use varies widely across countries:
- In North America, it’s mainly used in special forensic settings
- Some European countries have mostly stopped using it
- Professional guidelines provide rules for ethical use
- Insurance rarely covers PPG testing
- Most testing happens only in specialized centers
Best Practice Recommendations
For places that use PPG testing, best practices include:
- Clear informed consent procedures
- Strict protocols for testing and interpreting results
- Using PPG along with other assessment tools
- Regular equipment checks
- Strong privacy protections
- Testing done only by specially trained clinicians
Future Directions
The field continues to change in several ways:
Technology Advances
- More sensitive measuring devices
- Computer-run testing to improve consistency
- Better ways to analyze data
Research Needs
- Studies on reliability with larger groups
- Comparing results with other measures
- Looking at how culture affects responses
- Studying how well it predicts treatment outcomes
Ethical Evolution
- Better consent procedures
- Less invasive testing materials
- Balancing public safety with individual rights
Expert Perspectives
Dr. James Cantor, a well-known sex researcher, says: “PPG remains one of the few objective measures we have. But it should be part of a complete assessment, not used alone.”
Dr. Lynne Zuccato, former head of a treatment association, states: “Using PPG ethically requires extensive training and strict rules. When done right, it provides valuable information we can’t get otherwise.”
Frequently Asked Questions
Is PPG testing allowed in court?
It depends on the court system. Many US courts find PPG too invasive for general evidence. But some allow it in specific cases like civil commitment hearings or treatment progress reports.
How accurate is PPG testing?
Research shows moderate to good reliability when done properly. It can distinguish between groups with different known sexual interests. However, false results are possible. That’s why it works best as part of a complete assessment.
Can people fake their responses during PPG testing?
Some people can suppress their arousal through mental tricks. Complete control is difficult though. Modern tests include ways to detect deliberate manipulation.
Is PPG testing used outside legal settings?
While once used in research on sexual orientation and erectile problems, today it’s mostly limited to forensic settings and specialized treatment centers.
Are there PPG tests for women?
A female version called vaginal photoplethysmography exists. It’s used almost exclusively in research rather than clinical work due to technical challenges.
Conclusion
PPG remains a controversial but science-based tool. It has specific uses in forensic and clinical psychology. It offers unique insights into arousal patterns that self-reports might miss. But its invasive nature and ethical concerns limit its use to specialized settings.
The future of PPG likely involves combining it with less invasive techniques. For professionals in this field, understanding both what PPG can and cannot do is essential. For the public, knowing what PPG involves helps inform discussions about its proper role in assessment and treatment.