Why do people complain? Not to torture others with their negativity, surely. When most of us indulge in a bit of a moan, the idea is to “vent.” By getting our emotions out, we reason, we’ll feel better.

But science suggests there are a few serious flaws in that reasoning. One, not only does expressing negativity tend not to make us feel better, it’s also catching, making listeners feel worse. “People don’t break wind in elevators more than they have to. Venting anger is…similar to emotional farting in a closed area. It sounds like a good idea, but it’s dead wrong,” psychologist Jeffrey Lohr, who has studied venting, memorably explained.

OK, so complaining is bad for your mood and the mood of your friends and colleagues, but that’s not all that’s wrong with frequent negativity. Apparently, it’s also bad for your brain and your health. Yes, really.

On Psych Pedia, Steven Parton, an author and student of human nature, explains how complaining not only alters your brain for the worse but also has serious negative repercussions for your mental health. In fact, he goes so far as to say complaining can literally kill you. Here are three of the ways he claims that complaining harms your health:

1. “Synapses that fire together wire together.”

This is one of the first lessons neuroscience students learn, according to Parton. “Throughout your brain there is a collection of synapses separated by empty space called the synaptic cleft. Whenever you have a thought, one synapse shoots a chemical across the cleft to another synapse, thus building a bridge over which an electric signal can cross, carrying along its charge the relevant information you’re thinking about,” Parton explains.

“Here’s the kicker,” he continues. “Every time this electrical charge is triggered, the synapses grow closer together in order to decrease the distance the electrical charge has to cross…. The brain is rewiring its own circuitry, physically changing itself, to make it easier and more likely that the proper synapses will share the chemical link and thus spark together–in essence, making it easier for the thought to trigger.”

So let’s boil that down–having a thought makes it easier for you to have that thought again. That’s not good news for the perpetually gloomy (though happily, it seems gratitude, can work the opposite way, building up your positivity muscles). It gets worse, too. Not only do repeated negative thoughts make it easier to think yet more negative thoughts, they also make it more likely that negative thoughts will occur to you just randomly walking down the street. (Another way to put this is that being consistently negative starts to push your personality towards the negative).

Parton explains how these closer synapses result in a generally more pessimistic outlook: “Through repetition of thought, you’ve brought the pair of synapses that represent your [negative] proclivities closer and closer together, and when the moment arises for you to form a thought…the thought that wins is the one that has less distance to travel, the one that will create a bridge between synapses fastest.” Gloom soon outraces positivity.

2. You are whom you hang out with.

Not only does hanging out with your own negative thoughts rewire your brain for negativity, hanging out with negative people does much the same. Why?

“When we see someone experiencing an emotion (be it anger, sadness, happiness, etc), our brain ‘tries out’ that same emotion to imagine what the other person is going through. And it does this by attempting to fire the same synapses in your own brain so that you can attempt to relate to the emotion you’re observing. This is basically empathy. It is how we get the mob mentality…. It is our shared bliss at music festivals,” Parton writes. “But it is also your night at the bar with your friends who love love love to constantly bitch.”

The takeaway lesson is, if you want to strengthen your capacity for positivity and weaken your reflex for gloom, “surround yourself with happy people who rewire your brain towards love.” If you’re looking to deflect others’ negativity, here are a few tips.

3. Stress is terrible for your body, too.

All of which sounds like a good argument for staying away from negativity to protect your mental health, but Parton insists that quitting the complaining habit is essential for your physical health, too. “When your brain is firing off these synapses of anger, you’re weakening your immune system; you’re raising your blood pressure, increasing your risk of heart disease, obesity and diabetes, and a plethora of other negative ailments,” he says.

The culprit is the stress hormone cortisol. When you’re negative, you release it, and elevated levels of the stuff, “interfere with learning and memory, lower immune function and bone density, increase weight gain, blood pressure, cholesterol, heart disease…. The list goes on and on,” says Parton.