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Aggression Reinforced by Reward Mechanism
> 1/15/2008 1:56:23 PM

Violence can be extremely frightening, but a quick look at the most popular movies and video games will reveal that it can also be exciting. Those who engage in aggressive sports such as boxing often feel good after fighting, but it has not yet been established whether humans have a specific reward mechanism that reinforces aggressive behavior. Research from Vanderbilt University, published this week in Psychopharmacology, shows that such a mechanism does exist in mice, raising interesting new questions about negative patterns of human behavior.

The experimenters took one group of mice and habituated them to a cage so that they felt territorial enough to defend it. Then they set up a button in that cage that, when pushed, introduced an intruder mouse that could be easily attacked and driven away. The experimenters found that the at-home mouse purposefully hit the button many times after first accidentally triggering an invasion, seemingly a sign that it was enjoying the aggression that it got to dish out. Two more stages to the experiment confirm to a reasonable degree that this is the correct explanation.

The second stage of the experiment involved injecting the at-home mice with dopamine receptor blockers. These injected mice were much less inclined to hit the button, and when they did they were 50% less likely to try and bite the intruder. In other words, they weren�t getting the rush from attacking another mouse, so they didn�t bother calling for another victim.

The third stage ruled out one possible source of error by making sure that dopamine blockers do not dampen all activity rather than aggression exclusively. To settle this, mice were allowed to roam freely after dopamine blocker injections. There were no comparable changes in movement, demonstrating that it is not general lethargy that causes mice to stop pushing the button.

Dopamine is part of an incredibly powerful reward mechanism that directs humans towards food, sex, and other potentially beneficial and hard-to-resist goals. While this system often guides people towards activities that help them, it can quickly run out of control and reinforce destructive behavior such as gambling and drug abuse. This may be one explanation for why some people are seduced by violence. While some aggression can secure valuable resources for the aggressor, this behavior can become excessive if it is overly rewarded by extremely compelling neurotransmitters. While preliminary, this new experiment hints that aggressive patterns might be treatable with methods used for other addictive behaviors.

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