Behavioural traits such as tobacco and alcohol use can be regarded as intermediate traits, which are under a degree of genetic influence but which are themselves direct causal agents influencing various health outcomes. However, a critical difference between these and more usual intermediate phenotypes (such as LDL cholesterol) is that whereas both may be direct causal agents and amenable to intervention for therapeutic benefit, the former may be entirely absent (i.e., non-smokers, non-drinkers), whereas the latter cannot be (i.e., no one has a cholesterol level of zero). Genetic variants may influence whether or not someone smokes or drinks, or how much they smoke or drink, or both.
