findings: Zhou et al. (2008b) showed that SLC6A3/DAT1 was only associated with ADHD in the absence of conduct disorder. Another example is gender; given the difference between sexes, certain genetic risk factors can be expected to affect males and females differently. That this affects association studies has already been shown in ADHD (Biederman et al. 2008; Guimaraes et al. 2007; Rommelse et al. 2008b) as well as other (related) psychiatric disorders (Craig et al. 2004; Diamond 2007; Haefner et al. 2008; Kendler 1998; Lang et al. 2008; Okamura et al. 2007; Rucci et al. 2009). An additional potentially important factor is age at diagnosis. It is well established that age influences cognitive performance, with children performing worse in neuropsychological tests than adolescents and adults (Addamo et al. 2007) on tasks that are relevant to ADHD (Rommelse et al. 2008a). We and others have presented evidence that age can modulate the association of SLC6A3/DAT1 with ADHD, with different haplotypes of the gene being associated with ADHD in childhood and adulthood (Barkley et al. 2006; Elia and Devoto 2007; Franke et al. 2008).
