If there was a way of telling who in our society is sexually attracted to children, are we entitled to know? A recent study from Georg-August-University Göttingen in Germany suggests that we may need to grapple with this question.

Phallometric testing, also known as penile plethysmography, is considered the gold standard in measuring male sexual arousal, and particularly, deviant sexual interests such as pedophilia, which is the sexual interest in prepubescent children, roughly aged 3 to 10. The test involves measuring the volume of blood in the test-taker’s penis using an airtight glass tube (or conversely, measuring penile circumference with a mercury strain gauge) while he is presented with a series of images of children and adults, and audio stories describing a corresponding sexual encounter.

Phallometry is commonly used in forensic settings to assess the sexual interests of sex offenders, in order to determine their risk of re-offending. As one can imagine, sex offenders tend not to be forthright about their sexual preferences, which makes phallometry all the more important. It has, however, been criticized because the test can become easier for individuals to fool with each successive assessment.

Brain scanning using fMRI holds much promise as a diagnostic tool in evaluating sexual interests, as research has documented a reliable network of brain regions involved in sexual arousal. The current study took this another step by testing whether brain functional activation could be used to infer what someone finds sexually interesting without them knowing.

This was accomplished by presenting sexual images subliminally. Study participants were shown pictures very briefly (for 483 milliseconds, or about half a second), followed by another image, or “mask,” that served to disrupt communication between the retina and visual cortex. As a result, it wasn’t possible for viewers to manipulate their response because they weren’t consciously aware of what they had seen.

Twenty-four healthy, heterosexual, non-pedophilic men took part in the current study. All underwent fMRI scanning while viewing pornographic and nude, non-pornographic images of women and men. Half were shown masks that were pictures of inanimate objects; the other half saw masks that were scrambled versions of the image they had just seen.

The researchers did not find any fMRI effects for the group that saw the masks consisting of inanimate objects, but the group viewing scrambled masks did demonstrate activation patterns denoting sexual arousal, despite not recognizing what they had seen. This suggests it may be possible to covertly monitor what a person is interested in sexually, beyond their control or manipulation.

Some caution is in order because of the study’s small sample size and the use of uncorrected statistics, which do not rule out the possibility of false positives. The work does, however, offer promise for the application of fMRI to diagnose pedophilia, as well as other deviant sexual interests, such as biastophilia, the sexual preference for rape.

If such tests become possible, what then? Any such test is only ethical if its participation is consensual and test-takers are aware of what they will be shown. For example, if a patient has a past history of abusing children, we would need to take into account his likelihood of re-offending and the possibility that being exposed to clothed images of children, even subliminally, might increase this risk. Although it may be tempting to enforce testing in a variety of situations outside of a forensic context, we must first take a step back and ask ourselves what will be potentially lost and gained in doing so.

Michael C. Seto, the forensic research director at the Royal Ottawa Health Care Group, has estimated that 1% of the population is pedophilic, so statistically speaking, most of us probably know someone who is sexually attracted to children. It’s critical, however, that we recognize pedophilia and child sex abuse are two different things, because many pedophiles never act on their desires and don’t view child pornography because they understand that the effects of sexual abuse can be devastating.

Despite the fact that sexual interest in children was once believed to be the result of sexual abuse in childhood, current research suggests that pedophilia is biological as opposed to learned, and apparently immutable due to associated differences in brain structure and function. Furthermore, a recent fMRI study showed that non-offending pedophiles have greater inhibitory control than pedophiles who have offended, which likely explains why some are more successful in avoiding abusing a child.

As someone who has worked with pedophilic men in both research and clinical capacities, I believe early identification and prevention are critically important in reducing rates of child sexual abuse. This includes making support available to non-offending pedophiles so that they don’t ever offend. We must encourage an open, fact-based dialogue on this issue because an emotionally charged, righteous approach drives it underground. The reality is, avoiding uncomfortable conversations will only make our society less safe for children.