In states with stronger alcohol policies, high school students tend to report less drinking, even when the policies don’t target youth directly, according to a new study.

The findings support the Institute of Medicine, which says strategies to reduce and prevent underage drinking must target parents and adults, the authors wrote in the journal Pediatrics.

“This is the first study to examine possible effects of all state alcohol control policies collectively, and the subset of state policies most relevant to underage drinking,” said Mallie J. Paschall of the Prevention Research Center at the Pacific Institute for Research and Evaluation in Oakland, California.

Paschall, who was not involved in the new study, had previously compared alcohol policies by country and found similar results: lower levels of youth drinking in countries with stronger alcohol policies.

Some policies aimed more at adults may affect underage drinking indirectly, Paschall added by email.

For the current study, Ziming Xuan of Boston University School of Public Health and colleagues used a scoring tool called the Alcohol Policy Scale to rate the strength of state-level policy measures.

They also looked at data from seven Youth Risk Behavior Surveys of high school students conducted biannually between 1999 and 2011. Students in 9th through 12th grade reported how often they had any alcoholic drinks or been binge drinking over the previous month.

As state alcohol policy scale scores got higher, the odds of youth alcohol consumption and binge drinking declined, the researchers found.

This relationship was the same for youth-oriented alcohol policies and general alcohol policies that do not target youth. Stronger general alcohol policies were tied to less alcohol consumption per adult, and adult drinking was correlated with youth drinking.

Two policies aimed at adults - increased alcohol taxation and reduction in places where alcohol is sold - accounted for almost half of the policy-related reduction in drinking among adults, as reported in Pediatrics.

The expert ratings of alcohol policy strength were partly subjective, the authors admit. Also, they say, the scores didn’t account for federal or local policies, variations between states, and policies with insufficient data, which limits the results, the authors wrote.

“It's possible that other factors not measured in this study influence both the alcohol policies and underage drinking, so we can't conclude that there is a causal effect,” Paschall said. “However, the study did control for a number of other state and individual characteristics that could influence both alcohol policies and/or underage drinking.”

A minimum legal drinking age and minimum drinking age sales laws, which are aimed at youth, and increased alcohol taxation, which is not explicitly aimed at youth, appear to be most effective at curbing youth drinking, he said.

Stronger alcohol policies were tied to a seven percent reduction in odds of youth drinking and eight percent reduction in odds of binge drinking, which may not seem like a large reduction but does represent a large number of underage drinking occasions that did not happen, he said.

“Increasing alcohol taxation and enforcing minimum drinking age laws would be an effective strategy for reducing both adult and underage drinking,” Paschall said.