Donald Trump. Alex Wong/Getty Images Donald Trump's ascent from wealthy political outsider to Republican presidential nominee has confounded many Americans, as well as the political establishment.

But a neuroscientist says Trump's popularity can be explained by how he triggers certain emotions — anger, fear, and aggression — in the "fear center," which is part of the limbic system in people's brains.

"We have the same brain we had 100,000 years ago when we were living on the plains of Africa," R. Douglas Fields, neuroscientist and author of "Why We Snap," told Business Insider. "These defensive triggers exist for a good purpose but politicians are pushing on them to motivate people to do what they want."

Widespread modern fears — of terrorism, war, and gun violence — as well as economic uncertainty make people even more responsive these triggers.

Fields says Trump uses four main human instincts to get people's attention:

1. Being part of a tribe

"Any social animal is dependent on its group and will defend the group," Fields said. "We live only because we're part of a society."

Trump's at-times inflammatory rhetoric toward minority groups elicits tribal "them vs. us" instincts in the human brain. When people are told there is a threat to their tribe, their brain automatically tells them to defend it.

Fields says that many violent criminals are acting out of this type of "tribe" mentality. For instance, young people who don't have a stable family or community are more likely to join gangs, where they find some sense of belonging. The gangs then lash out at opposing tribes.

Republican presidential candidate Donald Trump speaks at a campaign event in Springfield, Illinois, November 9, 2015. Reuters/Jim Young

2. Threat to environment

Humans are fiercely territorial because "protecting family and home are basic instincts needed for survival," Fields says.

Trump's comments on immigrants play into this instinct, according to Fields.

Consider what Trump said at his presidential announcement in June 2015:

"When Mexico sends its people, they’re not sending the best. They’re not sending you … they’re sending people that have lots of problems and they’re bringing those problems with us. They’re bringing drugs. They’re bringing crime. They’re rapists. And some, I assume, are good people."

Fields says that by portraying immigrants as dangerous threats, Trump is inciting anger in people's brains. Saying he will build a wall is an attempt to protect that territory.

"Think of a cat: It might be friendly, but if you walk up to its food dish, he will snap," Fields said. "He's wired like many of us are to protect our resources."

When a human witnesses a trespasser, often his or her first reaction will be to turn violent.

A supporter of Republican Presidential candidate Donald Trump stands across the street from a protest held by a number of Latino organizations outside of NBC Studios on November 7, 2015, in New York City. Andrew Renneisen/Getty Images

3. Insults

Politicians have long used insults to get people fired up and on their side.

Trump's trademark use of monikers like "Crooked Hillary" (for Democratic nominee Hillary Clinton) and "Little Marco" (for former GOP primary rival and Sen. Marco Rubio) are meant to incite rage in his supporters.

"As a social species, we are dependent on rank in society," Fields said. "Rank, especially among males, is established through aggression."

Because people have stopped physically dueling over a disagreement, verbal sparring has taken its place.

"Trump's insults get people to rally with him," Fields said. "It's his mechanism of engaging."

Republican presidential candidate Donald Trump shows off the size of his hands as rivals Marco Rubio, left, and Ted Cruz, right, look on at the start of the Republican presidential-candidate debate in Detroit, Michigan, March 3, 2016. REUTERS/Jim Young

4. Life-or-death situations

Talking about life-or-death situations elicits an emotional response in most people.

"Almost anyone will defend themselves in what is perceived as a life-or-death attack," according to Fields.

In an article for The Daily Beast, Fields noted that the word "kill" was used 53 times during the December 15 Republican primary debate. This language was not used at all during previous GOP debates.

Here's a sampling:

"Ted Cruz: '…we will hunt down and kill the terrorists.' Donald Trump: 'These are people that want to kill us…' Trump also advocates killing family members of ISIS terrorists. Lindsay Graham: 'They’re trying to come here to kill us all…' Mike Huckabee: 'We have to kill some terrorists and kill every one of them….'"

Fields has a tip for voters who want to make rational decisions.

"Whenever you feel angry, you have to ask yourself if you're being manipulated," Fields said. "Let the moment pass and ask yourself if aggression or violence is really the right way to fix a situation."