In the case of some individuals, it seems that their brain is made by only one neuron for aggression. But this new research carried out at the Vanderbilt University and published in the journal Psychopharmacology has really surprising results: aggression can be more rewarding than sex, food and drugs, explaining the industry of millions of dollars around boxing, martial arts or K1.

"Aggression occurs among virtually all vertebrates and is necessary to get and keep important resources such as mates, territory and food. We have found that the 'reward pathway' in the brain becomes engaged in response to an aggressive event and that dopamine is involved," Craig Kennedy, professor of special education and pediatrics, .

"It is well known that dopamine is produced in response to rewarding stimuli such as food, sex and drugs of abuse. What we have now found is that it also serves as positive reinforcement for aggression." said co-author Maria Couppis.

The team kept a pair of mice (one male, one female) in one cage and five "intruder" male mice in separate cages. The replacement of the female by an an intruder male activated the aggressive behavior of the "home" male mouse, translated through tail rattle, sideways stance, boxing and biting.

The home mouse learned to poke a target using its nose if he wanted the intruder back. Surprisingly, the resident male consistently did this, appearing that the aggressiveness rewarded him. When the home mice received a chemical inhibiting their dopamine receptors, they were less interested in the encounters with the intruders.

The dopamine receptor inhibiting chemicals did not cause lethargy, that could have influenced the aggressive behavior. This is the first research finding a connection between aggression and dopamine receptors.

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