Cause and effect in brain development of paedophilia

 In brain connectivity, development, personality, social neuroscience

sadchild.jpegPhysorg reports about an interesting forthcoming MRI study linking paedophilia to regional changes in white matter. Analysing structural MRI using voxel-based morphometry, paedophiles were found to have significantly smaller white matter volumes in specific regions, as the abstract demonstrates:

The present investigation sought to identify which brain regions distinguish pedophilic from nonpedophilic men, using unbiased, automated analyses of the whole brain. T1-weighted magnetic resonance images (MRIs) were acquired from men who demonstrated illegal or clinically significant sexual behaviors or interests (n = 65) and from men who had histories of nonsexual offenses but no sexual offenses (n = 62). Sexual interest in children was assessed by participants’ admissions of pedophilic interest, histories of committing sexual offenses against children, and psychophysiological responses in the laboratory to erotic stimuli depicting children or adults. Automated parcellation of the MRIs revealed significant negative associations between pedophilia and white matter volumes of the temporal and parietal lobes bilaterally. Voxel-based morphometry corroborated the associations and indicated that the regions of lower white matter volumes followed, and were limited to, two major fiber bundles: the superior fronto-occipital fasciculus and the right arcuate fasciculus. No significant differences were found in grey matter or in cerebrospinal fluid (CSF). Because the superior fronto-occipital and arcuate fasciculi connect the cortical regions that respond to sexual cues, these results suggest (1) that those cortical regions operate as a network for recognizing sexually relevant stimuli and (2) that pedophilia results from a partial disconnection within that network.

Now, a few things strikes me odd in this analysis and interpretation. First of all, why is the comparison group nonsexual offenders? After all, that the crime is of a sexual nature is absolutely central to the present question, and especially that the sexual offender has been interested in children. The obvious choice would be to compare paedophilic sexual offenders to sexual offenders who had adult victims (typically a male offending a woman). Here, the act of sexual offence is similar between the two groups, while the sexual “object” is the vital difference. In the present study, any significant difference could just as well be explained by the nature of the crime as the sexual inclination of the subjects. It’s a classic case of poor control of confounding variables.

Second, I strongly dislike the over-interpretations offere in both the article and the news story. First, the authors find significant differences in the superior fronto-occipital and arcuate fasciculi, and link these regions to studies showing involvement in response to sexual stimuli. Following this, they suggest that paedophilia may occur due to a disconnection in this network. Just based on the reasons given in the previous section, these results may be interpreted just as well as brain alterations in sexual offence in general.

But more than this, if one just skims the literature on these regions (fasciculi), one can see that they have been implemented in language lateralization/function and hallucinations and delusions. So interpreting the differences as relevant to paedophilia is a long shot.

Furthermore, the physorg story suggest that this study:

challenges the commonly held belief that paedophilia is brought on by childhood trauma or abuse. This finding is the strongest evidence yet that paedophilia is instead the result of a problem in brain development.

This is a serious over-interpretation of the results. When understanding white matter (and any brain) changes during development, one should be cautious to claim that the changes observed are the mere cause of “brain development” and not experience-related phenomena. Here, we need to divide between two effects: neurogenetic and psychogenetic effects. Neurogenetic effects are, in this story, changes in the brain that are caused by biological factors. Age-related atrophy is a good example of this. The cause is the accumulation of junk within cells/neurons that eventually hinder cell division and function. Psychogenetic factors, on the other hand, are observed brain changes that are caused by behaviour, in its broadest sense. For example, if you learn to juggle, areas in the motor regions of the brain will alter their size and connectivity to a measurable extent. Likewise, London taxi drivers are known to have larger posterior hippocampi as a result of their prolonged training in navigation.

So in the case of paedophilia, observed changes in the brain cannot be said to support a brain-based (neurogenetic) interpretation, and to challenge psychogenetic causes. Rather, it has been suggested that many paedophiles have been subject to similar maltreatment when young. At the least, just because the brain shows a difference, one cannot conclude anything beyond this about causation.

As neuroscience enters the domain of social sciences, it holds the promise to both enlighten and naturalize these age-old discussions. However, the use of mere reporting and tailored interpretations are far from sufficient, and may even lead us astray in the goal to achieve a better understanding of these, and related, phenomena.


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