The goal of this dissertation was to evaluate proximate mechanisms that facilitate incest avoidance, and elucidate under what circumstances these mechanisms may fail, integrating insights from the fields of forensic and evolutionary psychology. To set the stage, Study 1 was a meta-analysis that examined differences between biological and sociolegal incest offenders on two major risk dimensions (antisociality and atypical interests). While sociolegal incest offenders were more problematic on some indicators of antisociality, these groups did not differ in atypical sexual interests. These findings suggest that current models of child sexual abuse may not be sufficient to fully explain incest offending. Studies 2 and 3 examined the viability of the Westermarck hypothesis (1891/1921) - that early physical proximity leads to incest avoidance - and the mediating role of disgust in father-daughter (Study 2) and brother-sister (Study 3) relationships. The primary hypothesis for these studies was that disgust toward incest would mediate the relationship between physical proximity and incest propensity or behaviour. The results of Study 2 did not support the Westermarck hypothesis among fathers. While physical proximity may not activate incest avoidance in fathers, disgust toward incest may still be a proximate mechanism. The results of Study 3 were consistent with the Westermarck hypothesis and the mediating role of disgust as an incest avoidance mechanism among siblings, and also suggest that moderators, such as sexual behaviour that could result in offspring, could influence the strength of this mechanism. These findings suggest that mechanisms responsible for incest avoidance may be different for fathers and siblings.