Good question!
Although genetics alone can explain a large fraction of alcohol dependence (up to 50-60% in some studies), the highest risk for alcoholism occurs when genetically-vulnerable individuals grow up around parents and/or siblings who abuse alcohol (a risky environment). For example, men whose parents were alcoholics have an increased likelihood of alcoholism even when they are adopted at birth and raised by parents who are not alcoholics. However, the risk is even higher when they are raised in an environment with excessive drinking.

In other words, repeatedly drinking too much alcohol does not have the same effect on everyone. One of the most accurate predictors of future alcoholism is a person's initial response to alcohol. People who are less sensitive to alcohol drink more to achieve the same level of intoxication and are much more susceptible to future alcoholism as a result.

Several genes in the brain that control sensitivity to alcohol have been identified and are being investigated using human and animal models in laboratories around the world.