In a unique study of four previously convicted adult male pedophiles (Mage = 33.8, SD 9.7 years), utilizing structural magnetic resonance imaging (MRI) and imaging genomics (neuroimaging combined with genetic analyses), the authors propose that small variations in genotypes are responsible for paraphilic phenotypic expression (Tost, Vollmert, Brassen, Schmitt, Dressing, Braus, 2004).
The candidate genes identified within this study are those responsible for the secretion of dopamine, plus a defective D2 A1 receptor allele.  The D2 receptor allele appears to be involved in the control of compulsive and impulsive behaviors, and the propensity for addictions.  Individuals with defects in these genes are functionally hypodopaminergic, and tend to seek out experiences which will enhance the amount of dopamine secreted (Tost, et al., 2004).
Individuals with obsessive-compulsive disorders, Tourette ’s syndrome, ADHD, and other reward-deficiency spectrum disorders also express similar genetic variations, however the effects on striatothalamocortical pathways is differentially expressed.  For instance, the motor loop of the striatothalamocortical network appears to be most affected in individuals with Tourette’s syndrome, while the limbic and prefrontal loops are implicated in the expression of pedophilic tendencies (Tost, et al., 2004).  
The four participants in this study were additionally assessed for response inhibition, working and visuo-spatial memory, abstract reasoning, global intelligence, alertness, and cognitive flexibility.  Participants scored within the normal range for intelligence, visuo-spatial memory, and alertness, however all scored well below normal for working memory, response inhibition, and cognitive flexibility.  MRIs revealed no abnormal neurophysical causes for these deficiencies (Tost, et al., 2004).
The authors conclude that the driving force behind pedophilia is the phenotypic expression of defective genes within the dopamine reward system and associated neural pathways, although they urge that this small study is insufficient and needs to be expanded (Tost, et al., 2004).

Whether or not allelic variation within the dopamine reward system and the D2 A1 receptor site should be considered defects is a matter for evolutionary biologists and philosophers to debate.  I would prefer them to simply be called variations or alternate alleles, as their origins may be rooted in very early human history.
However, this admittedly small study does give us an indication of a genetically related cause for pedophilia, and demands further investigation, particularly in view of the proliferation of treatment programs designed simply with behavior modification in mind.


Tost, H., Vollmert, C., Brassen, S., Schmitt, A., Dressing, H., Braus, D. (2004). Pedophilia:Neuropsychological Evidence Encouraging a Brain Network Perspective.  Medical Hypotheses, 63, pp. 528-531.