Regardless of the explanation, the findings suggest “that there are very long-term benefits to a higher drinking age,” Norberg said.

From the Vietnam era until the mid-1980s, many states allowed people to purchase alcohol at the age of 18. However, a federal law pressured states to boost the drinking age to 21, and all did with the exception of Louisiana, which finally followed suit in the 1990s.

In the new study, Norberg and colleagues looked at surveys of 33,869 people born in the United States between 1948 and 1970. They examined the records to see if there were differences in alcoholism and drug abuse rates depending on when states allowed individuals to buy booze.

The study findings appear in an early online edition of the December issue of Alcoholism: Clinical and Experimental Research.

After adjusting their statistics to prevent things like the ethnicity of the respondents from skewing the results, the researchers found that those who lived in states that allowed drinking before age 21 were 1.3 times more likely to have suffered recently from alcoholism. They were also 1.7 times more likely to have had a recent drug abuse problem.

Norberg said lower drinking ages might have a “peer effect,” since that makes it easier to find friends of one’s age to drink with. “If the drinking age is at 21, it will be a little harder to find some friends to go out with. You’ll probably drink less often and have a smaller number of drinks.”