When a car cuts in traffic, what makes some drivers shrug their shoulders and others fume with road rage, bashing the horn or worse?
Scientists say it's the "warrior gene," a controversial name for a genetic variation that research has shown to have an ugly side tied to violence, risk taking and aggression.
Found in one-in-three western men, it is literally a shorter, less active version of a gene allele on the X chromosome known as Monoaminine oxidrase A gene.
"In many, many studies it appears implicated in behaviors that look like they're related to physical aggression or some kind of conduct disorder," said Rose McDermott, a political scientist at Brown and Harvard universities, told ABC News.
McDermott repeated one of her own experiments, with minor variations, for a National Geographic documentary, "Born to Rage," which will air on Tuesday, Dec. 21 at 6 p.m. ET.
In the experiment, McDermott tested men whom she knew from previous testing either had the gene variant or did not. Some of the men were mixed martial arts fighters, others former gang members and also one Buddhist monk.
They had all been told that they could take home real money after performing a vocabulary test. One man, who was actually a hired actor, annoyed the group with ridiculous questions. He was a real jerk. Later, it was revealed that he had taken half of the money the participants had earned.
The men were then given the chance to spend their remaining money to punish him. Several men did, and many of them had the genotype that seems to be a factor for violence. McDermott concluded that the provocation had pushed them over the edge.
"There was one person who was like, 'Oh yeah, I want this guy to pay, I want this guy to hurt' ... He actually had the genotype," McDermott said.
But having the gene alone is not enough to predict a violent personality. The gene must be linked with a history of childhood trauma, she said.
"Child abuse, sexual abuse, sexual assault as a child...Things that were related to severe parental abuse, neglect, absence, illness, drug abuse, alcoholism in the family when you're young," McDermott said.
But even with that combination -- the gene variant plus childhood trauma -- that is not a guarantee that someone will be violent. There can be choice, as the National Geographic documentary reveals.
"One of the really important things to recognize is that we are the only animal in some sense capable of overcoming our evolutionary history. We can think about it. We can change our outcomes," she said.
In fact, a monk with the warrior gene who participated in McDermott's experiment chose not to hurt his scheming teammate.
"He had the predisposition but he made a conscious choice to engage in loving kindness and compassion as opposed to vengeance," McDermott said.
The genetic variant is located on the X chromosome of DNA and because men only have one X chromosome the connections to violence are seen more readily in men. Studies have also linked the variant to gambling, gang membership and weapon use.
But McDermott said there is so much more that determines a person's destiny.
"There is no such thing as a gene for aggression, there is no such thing as a gene for fear, or a gene for Republicanism. It's much more complicated," McDermott said.