Biological influences in pedophilia

Over most of the 20th century, “nurture” theories of pedophilia were much more popular than were “nature” theories.  Over the past decade, however, findings been reported that can be explained only by the presence of biological factors.

The first of these is the mean IQ of various groups.  IQ, in this context, might be thought of like blood pressure—a general indicator of overall health of the brain (instead of the heart).  Although they were not planning to conduct research with these data, we actually do have reports on the IQs of thousands of sex offenders. Starting in the 1930s, measuring IQ was often a part of a comprehensive health assessment, and such assessments were routinely conducted by various institutions, including prisons.  In order to find out whether there was any pattern, my team pulled together all available reports, pooling the samples into men who committed sexual offenses against children, men who committee sexual offenses against adults, men who committed nonsexual offenses, and men who had committed no known offenses (and an “other” category).  It was the samples of sexual offenders against children who were significantly lower in mean IQ across these studies.

The average range of IQ is 90–110 (a mean of 100).  The average IQ scores of all available samples of sexual offenders against children was XX.  A difference of 7 points is significant, statistically speaking, but still within the average range.  (See REF)

Another pattern also emerged from this analysis.  Although very many studies categorized people into “offenders against children,” they did not all use the same definition of “child.”  That is, some studies had “offenders against children under 18,” and others had “offenders against children under 16,” etc.  This showed a systematic pattern: The more stringent the definition of child, the lower the mean IQ of the sample.  That is, the more pedophilic the sample likely is, the lower its mean IQ.

(See REF)

Using the data archived in our own clinic, we found the same pattern for verbal memory tests, visuospatial memory tests, histories of failing grades in school (or being assigned to special education classes), having been knocked unconscious (before age 13), being physically shorter in height (by 2.5cm—roughly double the effect of smoking during pregnancy), and triple normal rates of non-right-handedness (the same range as observed in schizophrenia).  Because characteristics such as handedness exist prenatally (fetuses show a hand preference in thumb-sucking), whatever chain of events leads to the development of pedophilia, it had to have been present prenatally.

Imaging pedophilic brain structure

Early studies of the brain in pedophilia were conducted largely with CT scans and provided inconsistent results. The advent of MRI-based scanning dramatically increased the clarity of images, permitting more precise measurements. Our first MRI study used conventional (T1-weighted) scans of the brain and indicated that pedophilic men differ from men who committed non-sexual crimes in having less volume in large white matter regions. White matter is the brain tissue that connects brain regions, permitting those regions to function as a single network.


Three-dimensional renderings of the brain regions that are smaller in pedophilic than in nonpedophilic men. High resolution versions are in Cantor et al. (2008) Journal of Psychiatric Research, 42, 167–183.

We are currently gathering data, using more newly developed neuroimaging techniques to pursue this farther.  I am grateful to the Canadian Institutes for Health Research for enabling this work.

Functional neuroimaging of pedophilia (fMRI)

Another MRI technology, called functional Magnetic Resonance Imaging or fMRI, provides information on the brain’s reactions to different types of stimuli. In collaboration with researchers from the Toronto Western Hospital and Ryerson University, we are investigating the possibility to use fMRI to detect pedophilic sexual responses. This may lead to improved methods of diagnosing pedophilia. Piloting this project is made possible with the generous support from the University of Toronto Dean of Medicine’s New Faculty Grant.

The first step in this project has been to ascertain which brain regions respond to sexual stimuli in non-pedophiles:

Brain regions that respond to visual sexual stimuli in human males: Preliminary meta-analysis presented at the 2008 annual meeting of the International Academy of Sex Research, Leuven, Belgium. (Collaborators: Todd Girard, Matt Lovett-Baron, and Thomas Blak


© 2011 Dr. James Cantor