There are, however, two ways in which twins differ from singletons: (i) lower birth weight, due to shorter length of gestation (Plomin et al., 2001) and (ii) delayed language development (Rutter and Redshaw, 1991). Birth weight has been found to have a minimal effect on academic performance; for twins this effect was judged relative to what is a normal birth weight for twins and not for singletons (Christensen et al., 2006). However, studies have shown that children with birth complications are more likely to later develop antisocial and aggressive behavior (Raine, 2002), but birth complications may not by themselves predispose antisocial and aggressive behaviors, but will require the presence of an environmental risk factor (e.g., poor parenting, maternal rejection). In other words, the relationship between birth complications and antisocial and aggressive behavior is confounded by environmental risk factors (Hodgins et al., 2001; Raine et al., 1997).
