In genetic correlation analyses, the correlations of addiction risk and externalizing with external traits were largely similar in magnitude, although χ2 difference tests indicate that there were statistically significant differences in magnitude in 21 out of 84 traits tested. The greatest differences in the associations of externalizing and addiction risk were observed for tobacco-related phenotypes and other forms of psychopathology. For example, age of smoking initiation was more strongly correlated with externalizing than with the addiction risk factor, which is consistent with evidence from twin studies suggesting that genetic variance for general externalizing, rather than variance specific to substance use, predicts age of initiation.45 The finding that the addiction risk factor is more strongly associated with neuroticism, major depression and schizophrenia and less strongly associated with extraversion, suggests that what distinguishes SUDs from externalizing is not necessarily variance specific to risk for addiction, but rather risk shared with other forms of psychopathological distress. This may also reflect differences in ascertainment such that participants in the SUD GWAS were drawn from clinical populations and therefore likely have a higher rate of
