To what extent are the twin study results generalizable to the wider population? Twins and singletons have been found to experience similar rates of psychiatric disorders (e.g., attention-deficit hyperactivity disorder (ADHD), oppositional defiant disorder, conduct disorder) and behavioral and emotional problems (Gjone and Novik, 1995; Moilanen et al., 1999; Simonoff et al., 1997; van den Oord et al., 1995). Findings from the RFAB study show no differences in mean levels between MZ and DZ twins in reactive or proactive aggression (Baker et al., 2008; Tuvblad et al., 2009). It can, therefore, be assumed that twins and singletons display equal rates of aggressive behavior.
