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It’s the excuse all rude drunks have been waiting for: You might just be wired to be a jerk when you drink alcohol.

At least, that’s what the findings of a new Finnish study suggest. When researchers interviewed 156 peoples on their personality traits and alcohol history, they found that those who carried a certain genetic mutation—called HTR2B Q20—were more likely to have aggressive outbursts, get into fights, and act rashly while drinking.

Experts still have a lot to learn about how genes might affect the way that you react to alcohol. But it’s been proven that drinking tends to make everyone a little bit more impulsive. 

That’s because alcohol affects your brain in two ways. Along with altering certain neurotransmitter receptors to make you feel more relaxed or sedated, it also signals the release of higher levels of the chemical dopamine.

This activates your brain’s reward center, making you feel really good. So you become more willing to try things you might not normally do—like singing karaoke to Justin Bieber—and get a bigger high from doing it.

But the researchers found that HTR2B Q20 carriers have a smaller number of the brain receptors that play a role in controlling impulsive behavior, says study author Roope Tikkanen, M.D., Ph.D. 

So when they drink, they might be less likely to control the urge to, say, hit on their buddy’s girlfriend or make a snide comment to the 300-pound bouncer. 

However, that mutation is only thought to affect about 2 percent of the population. And it seems to be pretty exclusive to people of Finnish descent, says Dr. Tikkanen.  

So what’s the excuse for all the drunk assholes you run into on Saturday night at the local bar? 

Even though they might not have that particular gene mutation, they might be more prone to impulsive behavior, too. Maybe they have another genetic mutation that hasn’t been discovered yet. 

Or they could just be pricks.

It’s hard to know what exactly is driving the booze-fueled rude behavior. But it could be related to how unpleasant they are when they’re sober, says Rachel Winograd, Ph.D. (c), of the University of Missouri.

Alcohol may intensify some parts of your existing personality. In other words, someone who tends to be argumentative when he’s sober may be more likely to pick a fight when he’s drunk. 

But thanks to the boost in impulsivity, even people who consider themselves to be nice when they’re sober can turn into jerks when they’re drunk. 

In fact, Winograd’s recent study found that close to a quarter of people surveyed said they became less agreeable, less responsible, and less smart when they boozed compared to when they were sober.

If you find yourself becoming angry or belligerent when you’re drinking, it’s not something you should just ignore. This personality change can lead to dangerous or unsafe behaviors, and even compromise relationships in your life.

And that might mean you have a problem with alcohol—even if you’ve only had one or two drinks. 

The researchers can’t pinpoint the exact amount of alcohol that will cause a problem for you. It varies from person to person, depending on how it affects your behavior.

But Winograd suggests keeping close tabs on exactly how much you’re drinking, and the number of drinks it takes for your personality to be negatively altered. When you’re aware of this number, you can cut yourself off before you ever get to that point. 

Or, if hate the way you act when you drink, you can avoid it all together, she says. 

If you’re still having trouble controlling your actions while drinking, talk to a doctor or therapist. He or she can help you figure out a game plan to scale back or quit.