The interpretation of the analytical strategy used in this study is difficult. First of all, the direction of causation is not clear: it could be that maternal expressed emotion influences ADHD severity in the child, on the other hand, a child with severe ADHD might also elicit more pronounced criticism and less warmth from parents. Furthermore, explaining the biological effects of the environmental moderators of ADHD severity is not straightforward. The authors discuss epigenetic effects (like DNA methylation) as possible mechanisms. This might indeed be supported by at least one of their findings: PIWIL4 plays a role in chromatin-modification, another epigenetic regulator of gene expression (Szyf et al. 2007, 2008).
