Men get along better when there's drinking involved. (Justin Sullivan/Getty Images)

When bros share brews, they also start sharing smiles, according to a new study published in Clinical Psychological Science. That could explain why men are much more likely to drink in excess than women are -- they just have more fun.

Humans and some other animals experience something called "emotional contagion". Without realizing it, your emotional state is affected by the facial expressions and cues of the people you interact with. If someone is smiling genuinely, you're likely to "catch" that smile -- especially if they're a close friend or family member.

People often tout alcohol as a lubricant in social settings. To test this theory, researchers split 720 healthy social drinkers into groups of three -- with each one assigned either a vodka cranberry, a non-alcoholic drink, or a placebo drink (alcohol-free, but with vodka smeared inside the glass). The groups were introduced and served the drinks at regular intervals.

The researchers wanted to test how well these strangers bonded socially. They looked for genuine smiles (as opposed to polite ones, which are easy to weed out on camera) and observed how quickly they jumped from one person to another.

When there was alcohol around, people "caught" smiles more readily -- but only in all-male groups. In mixed groups and all-female groups, alcohol didn't make smiles any more viral than non-alcoholic and placebo drinks. But sober all-female groups had more contagious smiles than their male counterparts. Men had to raise their blood alcohol level to be as social and friendly as women were while sober.

The danger here is that most people think of "social drinking" as fairly harmless, lead author and University of Pittsburgh PhD candidate Catharine Fairbairn said in a statement. Excessive drinking in social settings is less likely to raise red flags than habitual drinking alone. But because men really do seem to enjoy social situations more as they consume more alcohol, these group settings might be where they develop their dangerous drinking habits.