Comparing people to animals seems to increasingly be a part of our political discourse.

When Roseanne Barr tweeted that former White House senior adviser Valerie Jarrett was the child of an ape, it came only a couple of weeks after Donald Trump called immigrant gang members, “animals.”

Trump has been a target himself: On the cover of its April 2 issue, New York magazine depicted the president as a pig.

As a psychologist who studies social attitudes and intergroup relations, I get a bit uneasy when I see these types of insults get normalized. At their core, they’re a way to dehumanize others – a practice that can have pernicious effects.

In a range of studies, psychologists have been able to show how dehumanizing messages can influence how we think about and treat people.

In one study, after researchers subtly primed participants to associate black people with apes, the participants became more likely to tolerate aggressive, violent policing of black criminal suspects. Another study exposed participants to metaphors comparing women to animals. The participants subsequently showed a spike in hostile sexism.

Dehumanization has also been associated with an increased willingness to perpetrate violence.

One set of studies found that men who showed stronger automatic associations between women and animals reported a greater proclivity to sexually harass and assault women. Other work has shown that those who dehumanize Arab people are more supportive of violent counterterrorism tactics: torture, targeting civilians and even bombing entire countries.

At its most extreme, dehumanizing messages and propaganda can facilitate support for war and genocide. It’s long been used to justify violence and destruction of minorities. We famously saw it in the Holocaust, when Nazi propaganda referred to Jewish people as vermin, and we saw it during the Rwandan genocide, when the Tutsi people were referred to as cockroaches. In fact, international nongovernmental organizations consider dehumanizing speech one of the precursors to genocide.

Why are dehumanization and violence so closely connected? As social creatures, we’re wired to empathize with our fellow human beings, and we get uncomfortable when we see someone suffering.

Once someone is dehumanized, we usually deny them the consideration, compassion and empathy that we typically give other people. It can relax our instinctive aversion to aggression and violence.

Studies have found that once a person has dehumanized another person or group, they’re less likely to consider their thoughts and feelings.

For example, Americans tend to dehumanize homeless people. In one study, experimenters asked participants to describe a day in the life of a homeless person, a college student and a firefighter. Respondents were much less likely to mention the homeless person’s emotional state.

Dehumanization can even affect our brains: When we look at people we’ve dehumanized, there’s less activity in the medial prefrontal cortex, which is the area of the brain responsible for social processing.

Roseanne might have claimed her tweet was nothing more than a flippant Ambien-induced barb. Some may have chuckled at New York magazine’s caricature of Trump.

But the pervasive use of dehumanizing language is a slippery slope that can ultimately cause tremendous harm – and that’s no joke.

Allison Skinner receives research funding from the Society for Personality and Social Psychology, the Society for the Psychological Study of Social Issues, and Northwestern University.