XYY Aggression Theory

It is generally believed that aggressive and violent behavior arises from the combination of hereditary influences and the effects of several environmental risk factors. However, some early researchers argued that criminal and violent behavior was mainly explained by hereditary factors. For instance, Cesare Lombroso, an Italian doctor, theorized that “born criminals” could be distinguished by atavistic stigmata (physical features) such as a large jaw, sloping forehead, or a single palmar crease. Later, in the 1960s, the relationship between heredity factors and criminality was highlighted by the so-called XYY syndrome. This came about because several studies had suggested that affected individuals commit criminal and violent acts more frequently than expected. Given that normal XY males display higher levels of violence and aggression than females, one plausible hypothesis is that having an extra Y chromosome might contribute to even higher levels of crime and violence in XYY males.