If you’ve ever had one (or ten) too many drinks at a bar, you’re probably familiar with this scenario: a drunk guy stumbles past you, spills a beer all over you, and you get angry. You’re convinced he did it on purpose, and you start fuming. According to a new study in Personality and Social Psychology Bulletin, you’ve probably fallen victim to one of the many side effects of booze: assuming that others' actions are intentional.

The 92 male participants in the study were led to believe that they were participating in a taste test. After fasting for 3 hours, each subject was given a cold juice drink. For half of the subjects, the drink was pure juice, but the other half were treated to a drink containing more than a shot of pure alcohol. Within each group, half the participants were told that the drink contained alcohol, and half were not. To complete the illusion, the rims of the glasses of those participants expecting a boozy drink were sprayed with alcohol just before serving.

Once they’d finished the drink and spent some time on unrelated tasks while the alcohol was being absorbed, the subjects were given a series of 50 action statements. Some actions could be interpreted as either deliberate or accidental (“He deleted the email”), some could only be deliberate (“She looked for her keys), and some could only be accidental (“She tripped on the jump rope”). For each statement, the participants had to choose whether the action was done intentionally or unintentionally.

Nearly all the participants, no matter what condition, judged all the unambiguous statements correctly. However, when the actions were ambiguous and could have been performed either intentionally or unintentionally, the "drunk" participants were much more likely to perceive the actions as deliberate than the sober participants were. The clever design of this experiment allowed the researchers to separate the actual physical effects of alcohol from its expectancy effects. What the subjects believed they had consumed didn't affect their responses—only whether they had actually consumed booze or not.

Alcohol makes you much more likely to believe that the guy that knocked into you in the bar is out to get you, instead of recognizing that he probably didn’t even see you in his drunken stupor. This biased way of thinking, or "intentionality bias," is a major factor in the link between alcohol and aggression. So, next time you get pissed off at someone after a few drinks, think twice before hitting the guy: he's probably even drunker than you are.

Personality and Social Psychology Bulletin, 2010. DOI: 10.1177/0146167210383044 (About DOIs).