Is Aggression Like Sex?

No wonder there's so much violence in the world. Scientists have found evidence that aggression rewards the brain in much the same way as sex, food and drugs.

Researchers at Vanderbilt University in Nashville found that the same "reward pathway" in the brain responds to aggression with the release of the neurotransmitter dopamine, known to be produced in response to various stimuli. Although the preliminary work involved lab mice, the scientists believe their findings probably apply to virtually all mammals, including humans.

"If I'm injected with cocaine, and I'm a mouse, my reward pathway activates. If I'm a human being and I'm injected with cocaine, that same reward pathway lights up," said Craig Kennedy, a professor of special education and pediatrics at Vanderbilt and co-author of a paper on the research, in the Jan. 14 issue of Psychopharmacology. Now, Kennedy and a colleague, Maria Couppis, have shown that the same reward pathway is involved in aggression as well.

The pair came up with a new way to test the theory that aggression feeds upon itself by stimulating the reward system.

The volunteers for the research included lab mice, a male and a female, called the "home mice," in one cage, and a bunch of other mice, all males, in another cage. The female was removed from the home cage, and a male intruder was allowed in. There followed an intense battle between the two males, with lots of tail rattling, boxing and biting.

After a few minutes of combat, the intruder was removed, and here's where the interesting part comes in. The male home mouse had been taught to "nose poke," a trigger to communicate that he wanted another piece of the intruder.

That action allowed the intruder to return, and another battle ensued, and so on into the evening.

But that, in itself, proves nothing, other than a mouse likes a little action now and then. However, the researchers came up with a technique to insert a tiny tube into the small area of the mouse's brain where the reward pathway is found. That allowed them to pump in a drug that suppresses dopamine in that small area of the brain.

The result was so obvious that anyone could see it just sitting next to the cage. The home mouse, now robbed of the dopamine and the excitement from aggression, was through wanting to fight.

He walked around the cage, and poked his nose into various places, like mice generally do, but he didn't poke the trigger that would allow the intruder to return.

"He had absolutely no motivation to do it," Kennedy said.

The exciting reward associated with aggression was kaput, finished, laid away.

What that means, Kennedy said, is the home mouse got some "positive feedback" from aggression, but when the dopamine was suppressed, the thrill was gone and he wanted no further combat. What's true for a mouse is not always true for humans, but there are remarkable similarities between the anatomy of a mouse brain and the brain of a human.

"There have been lots of studies on drugs, of abuse, and other rewarding stimuli, that showed that the same area [of the brain] is involved in human beings as in mice," Kennedy said.

It will take more experiments, involving human subjects, to prove that the Vanderbilt research holds for humans, as well, but there is reason to believe that we're not all that different.

Nearly all animals show aggression, especially males, and it is likely an important evolutionary product of the effort to survive.

"In just about any species you look at, aggression is part of their behavioral repertoire," Kennedy said. "It occurs for things like getting access to territory, to food, to mates, and protecting offspring."

Aggression has survived the long path of evolution, because it proved very useful along the way. And it still does for nearly all species, but among humans, it has become a bit of a problem. Too much aggression is a bad thing, and one of the ways we hide the fact that we enjoy a little combat, is to take part in sports — either as a participant or a spectator — that sends a little dopamine flashing through our brains.

"We sanction violence by watching violent sports, boxing, football and the like," Kennedy said. "That's an ideal way for us to enjoy watching aggression in a much more socially appropriate arrangement."

Kennedy plans to move his research into the human arena, but with a less-invasive technique. Instead of inserting a tube into the brains of his human subjects, he will use sophisticated new scanners to see what's going on inside the brains of his subjects.

All he has to do is get a few guys to sit inside a scanner, and watch a little boxing or football, engaging in a little vicarious aggression. He expects to have plenty of volunteers.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.