Donald Trump would not be US president without the votes of many, including Obamacare enrollees and immigrants, who are likely to find themselves worse off because of his policies. To some Democrat supporters, this widespread voting against self-interests can seem impossible to understand. To Molly Crockett, a neuroscientist and experimental psychologist at Oxford University, it makes perfect sense.

Crockett studies the psychology of punishment and has found that, rather than accept what they see as an unfair scenario, people will often choose to punish others—even at a personal cost to themselves. This desire to punish, she believes, can motivate those who feel they’re getting a bad deal to vote against the political establishment, regardless of whether the alternative is truly a better option.

The ”ultimatum game”

A classic way of studying the human desire to punish is through lab variations of the “ultimatum game.” In this set-up, one player is given money and has the option of splitting it with a second player. That second player can accept the portion of money offered or reject it. If they reject it, neither player gets a penny.

This experiment has been studied for close to 40 years and researchers have found that if the first player offers the second less than 30% of the total sum, most second players will see that as an unfair deal and reject it. They’ll forgo all money themselves in order to punish the first player.

Crockett sees the popular support for Trump and Brexit as real-world examples of the punishing behavior she observes in the lab. “Some of the expressed sentiments of voters in both the [Brexit] referendum and the US election did suggest there was a motivation to punish there,” she says. “That’s certainly not going to be the case for all voters, but quotations that I’ve read from some are consistent with the things people say in our experiments, when they’re treated unfairly and they prefer to punish rather than be at the end of a bad deal.”

Revenge and punishment can be addictive

Crockett’s experiments have worrying implications. Firstly, her brain imaging studies show that the act of punishing engages the part of the brain that signals reward. It’s the same brain area that’s hijacked by addictive drugs. “Certainly the raw ingredients are there, behaviorally and neurally, for expressing moral outrage to have an addictive quality,” says Crockett.

She also found that people often justify their actions by saying they were trying to teach a moral lesson, rather than because the act of punishing feels good. (This remains true even when a punishment is carried out in secret, and the recipient will never know of their punishment to absorb its lesson.) In retrospect, we tend to assign a moral motive to actions that are essentially vengeance.

“Data suggests that people are telling themselves and others that they’re punishing for moral reasons when in fact the motivations are more complicated than that,” Crockett says. “The motive to harm someone who you perceived has harmed you is a very strong force.”

So, for example, a desire to punish immigrants that you perceive as having taken something from you could be reframed as an effort to create safety. It will come as no surprise, given the fractured state of politics around the world, that people inflict harsher punishments on those from different social groups.

Stressful times and social media make us want to punish people

But if humans have always had this desire to punish, why would it only become so politically obvious in recent months? Crockett’s work shows that serotonin levels affect punishment motivations and, given other research suggesting that long-term stress affects serotonin production, it could be that stressful large-scale events (such as an economic recession) increase the desire to punish.

Social media is another troubling factor, with the potential to further fuel the desire for revenge. “I see social media as tapping into punishment motive in a couple of ways that may be harmful,” says Crockett. “We know that punishing engages the brain’s motivational circuitry and there’s an immediately gratifying aspect to punishment. When you express outrage on Facebook or Twitter, not only do you get the immediate satisfaction of posting that but you also get repeated and amplified reinforcement of that behavior because people like what you say, they share it, they re-post it—and it creates a highly self-reinforcing cycle.”

This behavior is apparent across the political spectrum, Crockett says. “I do see, on both sides, people getting sucked in to repeatedly shaming, expressing outrage,” she says. “This reinforces itself and it further drives people apart… I’m very worried about the way things are headed.”

There are no clear answers on how to reduce the desire to punish, but Crockett believes that being aware of this tendency can help temper knee-jerk acts of vengeance. Punishing others feels good in the short term, and we cannot ignore this uncomfortable truth. But if we don’t curb that tendency, the longterm consequences could be punishing for us all.