continuous in the extreme case). As few as, say, 3 or 4 levels may already require a challenging number of groups, especially if the moderator differs within twin pairs (i.e., is not ‘shared’), and the sample includes additional family members (e.g., parents, siblings, partners). In such circumstances, behavioural geneticists often turn to the moderation models proposed by Purcell (2002). The popularity of these model is evident given its frequent use in twin studies on moderation in the context of, for instance, cognitive ability (Bartels et al. 2009; Grant et al. 2010; Harden et al. 2007; Johnson et al. 2009a; Turkheimer et al. 2003; van der Sluis et al. 2008), personality (Bartels and Boomsma 2009; Brendgen et al. 2009; Distel et al. 2010; Hicks et al. 2009a, b; Johnson et al. 2009b; Tuvblad et al. 2006; Zhang et al. 2009), health (Johnson and Krueger 2005; Johnson et al. 2010; McCaffery et al. 2008, 2009), and brain morphology (Lenroot et al. 2009; Wallace et al. 2006).
