A fair amount of research in the past few decades has pointed to a genetic component to addiction. Specific genes related to the neurotransmitter serotonin have been linked to early-onset alcoholism, though it is unclear if this is related to alcoholism per se, or to poor behavioral control. In addition, Kenneth Kendler and colleagues have performed twin studies looking at the relative contribution of genes and environment to the abuse of, or dependence on, six different substances: cannabis (marijuana), cocaine, hallucinogens, sedatives, stimulants, and opiates. From their comparison of 1,196 male-male twin pairs, the authors concluded that on average, each form of drug addiction was about 55 percent attributable to genetics and 45 percent attributable to environment.

Moreover, there appeared to be a genetic vulnerability to developing drug addiction in general, but not to developing an addiction to any particular drug. Similarly, there was a general effect of environment, influencing the likelihood of any kind of drug abuse/dependence rather than abuse of a specific drug. Interestingly, opiate addiction seemed more heavily influenced by environment (78 percent) than genetics (23 percent). Perhaps opiates are less available than other drugs and, as a result, opiate abuse is heavily dependent on exposure to drugs in the environment.