We cannot exclude the possibility that unmeasured environmental confounders impacted both our substance disorder measures and educational attainment. Although we attempted to control for possible confounders, additional work is needed to confirm that this association holds in other populations and after control for additional variables. For example, cognitive ability has been implicated in both substance use disorders and educational attainment, although without prospective data it is impossible to tell whether low cognitive ability is a cause or consequence of substance involvement. Along this line, research by Latvala et al. (2009) has shown that verbal ability and psychomotor processing speed are reduced in individuals with substance use disorders, with evidence that educational attainment may mediate the association between verbal ability and substance use disorders.
