New research suggests that lowering the legal drinking age from 21 could boost the high school dropout rate.

Researchers looked back at high school dropout rates in the 1970s to mid-80s -- a time when many U.S. states lowered the age at which young people could legally buy alcohol. They found that when the minimum drinking age was lowered to 18, high school dropout rates rose by 4 to 13 percent, depending on the data source. Black and Hispanic students -- who were already more vulnerable to dropping out -- appeared more affected than white students.

"The minimum legal drinking age changes how easy it is for a young person to get alcohol," Andrew Plunk, lead researcher of the study, said in a statement. "In places where it was lowered to 18, it's likely that more high school students were able to get alcohol from their friends."

The findings do not prove that lowering the legal drinking age to 18 was the blame for the high school dropout rate. However, Plunk said, state drinking-age policies would likely be unrelated to the personal factors that put kids at risk of drinking problems or dropping out.

For certain kids, that access might lower their chances of finishing high school. Policies that allowed 18-year-olds to buy alcohol showed a particular impact on minority students, as well as young people whose parents had drinking problems. In that latter group, the dropout rate rose by 40 percent.

In the mid-1980s, federal legislation returned the legal drinking age to 21 nationwide.

However, there is an ongoing debate about lowering it again -- largely as a way to combat clandestine binge drinking on college campuses. The argument is that college students who can legally buy alcohol at bars and restaurants will drink more responsibly.

"I think this study gives us some idea of what could happen if we lower the legal drinking age," Plunk said. "It suggests to me that we'd see this same dropout phenomenon again."

The findings are detailed in the Journal of Studies on Alcohol and Drugs.