While he’s dropped clues throughout his entire presidential campaign, the full extent of Donald Trump’s vindictive nature has only been revealed recently. Not only does his obsession with revenge provide more evidence for the widely held belief that he is a narcissist, it further suggests that Trump has a psychological profile that could make him a very dangerous world leader. In fact, research has shown that people who score high on measures of vengeance—the intense desire to inflict harm on another person in order to ‘get even’ after a perceived injury or insult—typically lack empathy, possess uncontrollable anger, and are emotionally unstable.

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To describe Trump as a man who holds grudges would be a vast understatement. Last month in a blog post, the British billionaire and founder of the Virgin group, Richard Branson, briefly described his first encounter with Donald, something he called extraordinarily bizarre. At a one-on-one business lunch that occurred sometime in the distant past, rather than discussing entrepreneurship or ways to help the world, Trump spent the entire time explaining how he intended to dedicate the rest of his life to ruining the lives of five people who refused to help him after his company filed for bankruptcy.

Also last month, during a public appearance, Trump vowed to sue all 12 women who have recently accused him of sexual misconduct as soon as the election is over—which might seem not so unreasonable had he not explicitly bragged about such behavior in 2005. But Trump didn’t stop there. He threatened to sue The New York Times for publishing the women’s stories, and if his lawsuit history tells us anything, it’s that these are unlikely to be just empty promises. And these examples are just the tip of the iceberg.

USA Today found that Trump has been involved in over 3,500 legal actions in the last 30 years, many of which were cases filed by him. As reported by The Atlantic in the article, “The Lawsuits of Donald Trump,” over the last decade the billionaire has sued “a book author, a MISS USA contestant, a comedian, and global financial institutions.” That comedian was actually HBO talk show host Bill Maher, who promised to give Trump $5 million if he could provide proof that his father was not an orangutan. Luckily for Maher, the case was ultimately dismissed. Regardless, if you think that Donald’s vindictiveness is superficial, with lawsuit threats being simply how he ‘lets off steam,’ his track record clearly shows otherwise.

Given the anecdotal and objective evidence, there’s just no doubt that Donald is an extremely vengeful person, and it is unlikely that even the most loyal Trump supporter would deny that. In fact, given the intense hatred toward the Obama administration and the U.S. government felt by your average Trump enthusiast, Donald’s ambition to tear down the establishment that has continuously mocked him is likely a main reason why they love him.

Psychology research shows that vindictiveness is strongly correlated with narcissism, a personality trait that countless psychologists say Trump shows all the signs of having. Studies have also found that those who seek revenge are often very energetic and have dominant personalities. Further, vengeful people often seek powerful positions that allow them to inflict what they see as well-deserved hurt on their enemies. That pretty much describes Trump to a T.

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What might be the most troubling aspect of a vindictive president is the severe lack of empathy. Since empathy is the ability to see events from another’s perspective, and to share their experiences and emotions, it is an essential feature for a president who cares about the people, as the overwhelming majority of them do not live a life anything like his. Minorities, as well as the poor, elderly, and disabled, would all be negatively affected by an unempathetic leader. To make matters worse, vengefulness is strongly associated with anger and aggression, which can go unchecked with empathy deficits, resulting in—for example—brutal military decisions made with no regard for civilian lives abroad.

Another serious drawback of a vengeful president is that they would spend so much time and effort getting even that they would not have time to actually do the jobs that presidents must do. Under a Trump presidency, many important duties—like working with Congress on the budget and running the armed forces—would either be neglected or allocated to others while he plots and executes his revenge.

We can be sure that if Donald Trump becomes president, he will be dedicating a substantial amount of time to figuring out some sort of way to punish the media, which he feels has treated him very unfairly during the election cycle. In an episode of The Daily Show that aired last week, Dana Bash, a journalist and CNN reporter, described to Trevor Noah the chilling way Trump already directs anger and aggression toward the media at his rallies:

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“There is nothing as jarring as being in the pen—because the press has to be in a pen at these rallies, for security and for other reasons—and there are thousands of people between you and the candidate. And it’s a huge crowd. So Donald Trump will say, “Look at those people, they are horrible. They are scumbags. They are the worst people. They’re so dishonest.” And all of a sudden, in unison, every single one of the people will turn around and stare at us, and it’s very creepy and scary—it really is.”

What’s even scarier is imagining what orders he’d give people and how strongly they’d respond if he were actually President of the United States. And how could citizens be arrested and held in jail for following the president’s orders?

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We can also bet that Trump would immediately devote much of his limited resources into destroying every bit of Obama’s legacy, regardless of whether or not it would hurt the country. It would serve as payback for the 2011 White House Correspondents’ Dinner, where Barack Obama roasted Trump with clever zinger after zinger. Although the jokes were light-hearted and typical of the event, Trump’s largely stone-faced reaction showed that he likely felt humiliated, and there’s absolutely nothing that infuriates The Donald more than that. Even if Trump knew for sure that fixing Obamacare would be easier and less costly than starting from scratch, he would still tear it down just out of spite. He’s already promised to do this. A leader that operates like that is dangerous for the country because they are more concerned with revenge than the welfare of citizens.

While it might seem obvious that as president Trump would put effort into getting back at the Clintons and other Democrats, it is more likely that he will have his vengeful eyes directed at his own party. With the Clintons defeated, he’d be more motivated to focus on settling old scores with Republicans like Speaker of the House, Paul Ryan, who went against tradition by refusing to show support or enthusiasm for the party’s nominee. This greatly embarrassed and upset Trump, who has been attacking Ryan ever since. Ryan is just one of many “establishment Republicans” who expressed disapproval of Trump and are likely to now be on his disloyal list. Because that list is one that Donald takes very seriously, many Republican politicians have as much reason to fear a Trump presidency as Democrats do. This time around they would be wise to vote blue, as their careers and future may depend on it.

But a vindictive president is the biggest liability when it comes to the world stage. It is here where a vengeful Trump has the potential to do the most damage. It is a fact that the U.S. is despised by a number of nations with formidable military force, and that is not going to change no matter who is elected president this November. If it is Trump, it is only a matter of time before the brash billionaire feels he has been deeply insulted or disrespected by another world leader. And since research shows that vengeful responses are generally greater in force than their provocation, we could easily find ourselves in a quickly escalating conflict with North Korea, Iran, China, or Russia. Sure, there may be a budding bromance between Vladimir Putin and Donald right now, but just like Trump’s friendship with Ted Cruz, a falling out is inevitable. And when that occurs, we can again expect it to get really ugly really quick.

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Trump expects flattery, and that’s something he’s just not going to get a lot of as President of the United States. As a result, many of his actions and decisions will be driven by aggression and vengeance, which have the power to override logic and reason. Will he try to order the people to stand up and cause chaos if Congress or the Supreme Court makes a decision he doesn’t like? Would he get us unengaged in a nuclear war if he’s mocked and ridiculed by a foreign nation? We should all hope that these are questions that we’ll never know the answers to.

Bobby Azarian is a neuroscientist affiliated with George Mason University and a science writer. His research has been published in journals such as Cognition & Emotion and Psychonomic Bulletin & Review, and he has written for The Atlantic, The New York Times, BBC, Scientific American, Psychology Today, Slate, The Daily Beast, and The Huffington Post. He also runs the website Science Is Sexy. Follow him @BobbyAzarian.