Narrator: Why does alcoholism run in families? This is Science Today. In the 1980s, Dr. Mark Schuckit of the University of California, San Diego tested over 200 sons of alcoholics to see how sensitive they were to alcohol. He found that 40 percent of them were unusually insensitive -- meaning they had to drink more than the average person to get drunk. When he looked at the group ten years later...

Schuckit: What we found was, the level of response to alcohol at about age 20 was a very powerful predictor of alcoholism 10 years later. Not perfect, but very powerful. If you were a son of an alcoholic, and showed a low level of response to alcohol, your risk for alcoholism almost a decade later was 60 percent. If you were the son of an alcoholic and showed a high level of sensitivity to alcohol, your risk for alcoholism 10 years later was about 15 percent, a huge difference.

Narrator: It may be another clue to the riddle of why children of alcoholics are at greater risk themselves. For Science Today, I'm Steve Tokar.