It seems plausible that environmental factors, such as accessibility (availability and cost), societal, familial and peer norms and values, are more important in substance initiation. For example, in the United Kingdom the great majority of people first drink alcohol during their teens and consumption peaks in the late teens and early 20s [9,39]. Those who do not drink will usually not do so for cultural reasons, such as religious faith. For those without such cultural reasons to abstain, some alcohol consumption is ‘normal’. The increased importance of genetic factors with heavier use may reflect biological processes involved in the adaptation and habituation of the brain and body. Animal studies have indicated that the process of addiction depends on the dysregulation of neurochemical mechanisms in specific brain reward and stress circuits [40,41].
