In our study, the strength of the negative relationship between alcohol dependence polygenic risk and digit symbol coding and Mill Hill Vocabulary scores was attenuated when social deprivation and education were added as covariates. This may be because education and social deprivation are strongly correlated with Mill Hill Vocabulary, and much of the variance in Mill Hill Vocabulary is removed when adjusting for these variables. The causal direction is moot: higher verbal ability might be the result of more education and living in a more affluent area, or vice versa, or influences might flow dynamically in both directions (Deary & Johnson 2010). It is important to recognize that when education is added as a control variable it is not just as an environmental factor, partly genetically influenced cognitive ability also drives educational attainment to some degree.
