With the help of an MRI scanner and some child pornography, a new study claims to be able to tell whether someone is a paedophile: Assessment of Pedophilia Using Hemodynamic Brain Response to Sexual Stimuli.
It was an fMRI study of 24 self-identified paedophiles (recruited through a clinic offering anonymous treatment) and 32 male controls. Everyone was shown a series of images of naked men, women, boys and girls. The neural response to child vs. adult images was the main outcome measure.
Respect to the authors for getting that past the ethics committee.
The blob-o-grams above show that the paedophile's brains reacted differently to the control brains, when shown images of naked children, which is not surprising because the brain is what makes you a paedophile (and everything else.)
However, what's more interesting is that by comparing each individual's brain activity to the average activity of the paedophile group and the control group, it was possible to diagnose people as paedophiles or not with high accuracy (90+%).
Plotting the "typical paedophile"-ness of the neural response to girls vs women and boys vs men, the paedophiles (triangles) form a clear cluster. There were also some differences between homosexual and heterosexuals in both groups.
The statistics seem kosher: they used leave-one-out cross-validation to avoid the error of double dipping.
What's not clear is whether this was measuring sexual attraction as such. All it's measuring is how much each person's activity correlated with the paedophile group average. Maybe it's picking up on the shame paedophiles feel over being reminded of what they've done. Maybe the controls were just averting their eyes when the child porn came on.
However, you could say that if you're just interested in the practical business of catching paedophiles, that's academic. More concerning is the question of whether it would be possible to fool the technique. A recent study showed that it's easy to fool a brain scan designed to detect lying.
But let's suppose it does work out. Would that be a good thing? What is "a paedophile", anyway? Is it someone's who's attracted to children, or someone who acts on that attraction?
For example, there are people who are caught with child porn, and who admit they downloaded it, but who deny being attracted to children. The Who shredder Pete Townsend and comedian Chris Langham being two British examples. Both admit downloading illegal images, but say it was for 'research purposes'.
Now it might be possible, using fMRI, to find out if they're telling the truth. Let's suppose it was doable.
So what? Downloading child pornography is a crime - whatever your motivation. Being attracted to children is legal, in itself. So from a legal perspective it should make no difference at all in cases like this.
Of course, we don't in fact go around seeing things from a purely legal perspective. We care whether someone is attracted to children or not. But should we care? Is that fair? You don't choose your sexual orientation. What you choose is whether to break the law by commiting the crime.
There are surely people out there - no-one knows how many - who are attracted the children, and never act on it. Do we want to be able to "catch" them?
Edit: The original version of this post linked to the wrong paper, an older paper by the same authors. This has been fixed now.
Ponseti, J., Granert, O., Jansen, O., Wolff, S., Beier, K., Neutze, J., Deuschl, G., Mehdorn, H., Siebner, H., & Bosinski, H. (2011). Assessment of Pedophilia Using Hemodynamic Brain Response to Sexual Stimuli Archives of General Psychiatry DOI: 10.1001/archgenpsychiatry.2011.130