Bottoms up? Then it’s grades down, says a study co-authored by a UC Davis researcher.

Scott Carrell, an assistant professor of economics, and two co-researchers found that alcohol consumption "significantly reduces academic performance" enough to push a final class grade down by about a half of a letter grade. What's more, the negative effects are largest for highest-performing students.

Carrell, Mark Hoekstra from the University of Pittsburgh and James West of the U.S. Air Force Academy in Colorado conducted a six-year review of the grades of 3,884 students in mandatory courses at the academy.

Because the academy strictly enforces its ban on underage drinking, researchers were able to zero in on a narrow time frame of alcohol consumption — drinking on and after the 21st birthday. The researchers found that students who turned 21 just prior to taking standardized final exams performed at a lower level than students who turned 21 just after finals.

Also, the largest negative effects were for students who had been in the top half of their class by grade point average at the end of their freshman year.

Carrell says researchers found that the effect was persistent, rather than a one-time occurrence caused by students who party around their 21st birthday.

"If it were a birthday effect you would expect the negative effects to go away the next semester — but they don't," says Carrell. "Six to nine months later, student performance is still hindered."

He theorizes that the downturn comes from the distraction posed by partying, rather than the actual presence of alcohol in the students' systems during class and exams. "Students at the Air Force Academy aren't showing up to class drunk — they would get expelled," observes Carrell. "My sense is that [partying] diminishes study time and effort."

As an economist, Carrell focuses on how variables affect students' academic performance and, ultimately, their academic achievement. He compares the ill effects of student partying to the difference between having a top-notch professor versus an average one teaching a student's class.

The study, "Does Drinking Impair College Performance?", was published as a working paper by the National Bureau of Economic Research.