What makes a person physically or verbally hostile may be at least partially rooted in their genes, according to a new study.

Previous studies have established that levels of the chemical serotonin (search) in the brain relate to aggressive behavior and lack of impulse control (search).

Aggression is one of the most studied aspects of human personality, and researchers say about 30-60 percent of personality traits appear to have some genetic basis.

One gene that turns out to be associated with anger, aggressiveness, and impulsivity is the TPH gene, which helps regulate levels of serotonin.

The study shows that verbal and physical aggression are linked to variations in the TPH gene, which may play a role in depriving the brain of the mood-lifting chemical serotonin.

Aggression May Be in the Genes

In the first part of the study, researchers looked at the effects of increasing levels of serotonin in the brains of men who displayed two different forms of aggression: irritable, verbally abusive “neurotic hostility” and more violent, physical forms of “aggressive hostility.”

Each of the men took an antidepressant (search) that made serotonin more available to their brains.

The results showed that only the men who scored high on measures of aggressive hostility released large amounts of the stress hormone cortisol after taking the antidepressant.

The antidepressant should result in an increase in the cortisol; however, men with high aggressive hostility scores had significantly higher levels of cortisol compared with men with low aggressive hostility scores.

Researchers say the findings suggest that the brains of the physically hostile men were deprived of serotonin, which made them extremely sensitive to the serotonin-boosting effects of the antidepressant and primed them to overreact, in part by producing more cortisol.

In the second part of the study, which appears in the current issue of Behavioral Neuroscience, researchers compared three variations of the TPH gene in a group of 58 people who completed questionnaires on aggressive traits.

They found that one variation was associated with the highest levels of physical aggression and hostility while another was associated with the lowest levels. But none of these variations was associated with verbal aggressive traits.

Researchers say the findings suggest that genetic factors may contribute to personality traits like aggression both directly and indirectly by affecting biological processes, such as serotonin production.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Hennig, J. Behavioral Neuroscience, March 2005; vol 119: pp 16-25. News release, American Psychological Association.