A total of four studies (Goldberg et al. 2003; Stefanis et al. 2004; Bruder et al. 2005; Bertolino et al. 2006) published between 2003 and 2006 and comprising six independent samples were identified by the search strategy and met the inclusion criteria (Table 5). Where data were not available in an appropriate format for inclusion, we attempted to contact authors directly.
