Like most aspects of human behavior, the development of substance use disorders (SUDs) involves a complex interaction between nature (genetics) and nurture (e.g., family and school environments, peers). It is difficult to disentangle the unique of effects of both nature- and nurture-related aspects in the development of SUD. Because families share genes as well as environments, investigators have difficulty knowing which outcomes are a result of which causes. For example, if a man has an alcohol use disorder and his son subsequently develops the same, is that because the son was genetically predisposed to crave and respond to alcohol in this way, or because he watched his father drink alcohol during his childhood and he then mirrored that behavior himself? Substance use in general is a particularly distal outcome. However, genetic studies targeting more proximal behavioral outcomes have identified specific genetic markers (e.g., DRD4, DRD2, 5HTTPLPR), which influence neurochemicals in the brain (dopamine and serotonin) and influence individuals’ levels of novelty/sensation-seeking or behavioral disinhibition (both of which are associated with increased likelihood of substance use). However, not all people who try a substance develop an SUD, making it hard to make direct connections between specific genetic patterns and substance use disorders/addiction. Other studies, rather than trying to predict behavioral outcomes from a specific gene, employ methods such as twin studies (following identical twins reared in different families) to identify the extent to which SUDs can be attributed to genetic influences. Estimates from these types of studies suggest that about 40%-60% of the variation in SUD can be attributed to genetic heritability.