Some people may be biologically vulnerable to particular drugs. For example, evidence accumulates that heredity influences some aspects of substance use problems, especially those appearing by early adulthood (Crabbe, 2002):
- Twin studies. If an identical rather than fraternal twin is diagnosed with alcohol use disorder, the other twin is at increased risk for alcohol problems (Kendler et al., 2002). In marijuana use, too, identical twins more closely resemble each other than do fraternal twins.
- Genetics research. Researchers have identified genes associated with alcohol use disorder, and they are seeking genes that contribute to tobacco addiction (Stacey et al., 2012). These culprit genes seemingly produce deficiencies in the brain’s natural dopamine reward system: While triggering temporary dopamine-produced pleasure, the addictive drugs disrupt normal dopamine balance. Studies of how drugs reprogram the brain’s reward systems raise hopes for anti-addiction drugs that might block or blunt the effects of alcohol and other drugs (Miller, 2008; Wilson & Kuhn, 2005).
- Adoption studies. One study tracked 18,115 Swedish adoptees. Those with drug-abusing biological parents were at doubled risk of drug abuse, indicating a genetic influence. But then those with drug-abusing adoptive siblings also had a doubled risk of drug abuse, indicating an environmental influence (Kendler et al., 2012).
So, what might those environmental influences be?