Another important difference between the present study and previous ones is the measure used to assess the behavioral phenotype. As dependent variable, we used the physical aggression scale of the Aggression Questionnaire [22], a continuous measure that assesses individual propensity toward aggression and that can be administered to individuals in the normative range of aggressive behavior. In contrast, most previous studies have used relatively indirect measures of aggressive behavior, such as incarceration, criminal conviction, or a diagnosis of antisocial behavior or conduct disorder. The problem with these measures is that antisocial behavior or conduct disorder include a number of behaviors other than aggression and that they are dichotomous (presence or absence of the behavioral phenotype), which may lead to a corresponding loss of discriminatory power. Our findings suggest that the use of psychometric tools assessing behavioral aspects of aggression ranging from normative levels to pathological extremes may increase the likelihood of detecting interactions of MAOA genotype with early traumatic events.
