Study: Humans Crave Violence Just Like Sex

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Scientists have always been aware that mice as well as other animals have been drawn to fights, but until now scientists never knew the brain was involved.

In the study, researchers placed one male mouse and one female mouse in a cage. Then, the researchers removed the female and allowed another male mouse intruder into the cage. This move caused aggression in the male resident of the cage. The forms of aggression were noted as tail rattle, an aggressive sideways stance, boxing and biting.

After the fighting stopped between the two, the male mouse resident was trained to nose-poke a target to get the intruder to return. As a result, the mouse constantly poked the target and fought the intruding mouse. Researchers say this indicates that the aggressive behavior was seen as a reward.

"We learned from these experiments that an individual will intentionally seek out an aggressive encounter solely because they experience a rewarding sensation from it," Kennedy said.

In order to determine whether the brain had anything to do with the aggressive behavior, scientists blocked the dopamine levels in certain parts of the resident mouse's brain. Dopamine is known to be involved in rewards like food and drugs.

The researchers found that the treated resident mouse was less likely to act in an aggressive manner towards the intruder's entry.

“This shows for the first time that aggression, on its own, is motivating, and that the well-known positive reinforcer dopamine plays a critical role," Kennedy said.

The scientists say that the experiments have implications for humans because human and mice brains are very similar.

He added, "It serves a really useful evolutionary role probably, which is you defend territory; you defend your mate; if you're a female, you defend your offspring."