It doesn't take a scientist to determine that many young people who leave home for college begin drinking to excess, smoking and even using illegal drugs. But a new, long-term nationwide study shows that alcohol and illicit drug abuse drop sharply after marriage, while the tobacco habit proves far more tenacious.

Researchers from the University of Michigan found that becoming engaged, getting married and having children are all associated with a steep drop in use of alcohol and illicit drugs -- a phenomenon the researchers called the "marriage effect."

Forty-one percent of single subjects in the study reported heavy alcohol use in the two weeks before the survey, while 28 percent of the married subjects reported such behavior. Cocaine and marijuana use registered similar steep declines after marriage. Couples who live together without becoming engaged or marrying do not appear to clean up their lifestyles, the researchers found.