Whether it’s a bar brawl among strangers or a caustic argument between friends, in many cases, alcohol is to blame. And sure, we all know that drinking reduces inhibitions and can increase aggression. But what’s really going on when people imbibe that causes them to behave so regrettably?

A new study looked into drinkers’ brains for the answer—and revealed that alcohol temporarily “uncouples” important brain regions so that they can’t work properly together. The result: Drinkers literally lose the ability to accurately read other people’s facial expressions—and that can set the stage for all kinds of trouble. Here’s why people who have been drinking can’t “see straight”…

THIS IS YOUR BRAIN ON BOOZE

The study participants were all heavy social drinkers but not alcoholics. Each participant underwent two testing sessions. At one session, the person was given a beverage (the amount based on body weight) made of a flavored drink mix with 16% alcohol (the exact amount of alcohol varied depending on the person’s body weight). That’s a bit stronger than most wines and typically the participant wound up with a blood alcohol level of about 0.09%, which is slightly above the legal limit for driving for people 21 and over. At the other session, each participant was given a placebo beverage that had the same ingredients but just a tiny bit of alcohol (1%) to make it taste and smell nearly the same as the alcoholic drink, but without the intoxicating effects.

A few minutes after finishing the drink, each participant had a brain scan called a functional MRI (fMRI) that shows, in real time, exactly what’s happening in various parts of the brain. During the scan, the person completed an emotional face assessment task. This involved looking at a series of photos showing various facial expressions—fearful, angry, happy or neutral—and matching the faces that reflected the same emotion.

Results: Compared with when they were sober (after consuming the placebo drink), the participants had significantly more trouble recognizing the matching facial expressions that depicted fear or anger after they had consumed the alcoholic drink. But what really intrigued the researchers were the fMRI findings. The scans revealed that alcohol intoxication reduced the interaction, or “coupling,” between the amygdala (a brain region responsible for reading social cues) and the orbitofrontal cortex (the region used in processing social and emotional information and in making decisions).

Normally, the interaction between these two areas flows both ways in a dynamic, interactive manner, enabling a person to generate appropriate emotional responses. When these two brain regions are uncoupled by alcohol intoxication, this flow of information is interrupted…and a drinker loses the ability to read certain facial expressions accurately and respond appropriately.

This disconnect is problematic because facial expressions of fear and anger indicate that there is a threat—either in the environment (fear) or from the other person (anger). When an alcohol-addled brain cannot properly process these nonverbal cues, it’s little wonder that a drinker can wind up getting himself into trouble. Keep that in mind whenever you imbibe!

If you need help cutting back: Read “Drink Too Much? Have a Drug Problem? Warning Signs and What to Do.”