Individuals with the lowest quartile of parent monitoring was associated with a significantly increased risk and there was no gradient in the remaining three quartiles. This is consistent with a prior study showing only the lowest quartile level of parent monitoring was associated with a sustained higher risk of drug initiation (24). Furthermore, when we tested the interactions between the risk genotype and each of the quartiles of parent monitoring, we found a similar non-linear effect showing the interaction only between the risk genotype and the lowest quartile, without gradient in the remaining three quartiles.
