Why do men fight? For centuries, philosophers have pondered this question. Some have suggested that aggressive behaviour is part of the male make-up; others claim that fighting is a socially inspired behaviour, a belief that has led to a wide range of child-rearing tactics. Poets and writers of great literature may be disappointed to learn that the aggressive male passions that have caused duels, skirmishes, and wars are the result of reduced levels of serotonin in the brain. Why men? A group of researchers from the University of Akron state the Y chromosome that determines “maleness” (as opposed to the XX in females) governs serotonin levels. When stimulated, serotonin decreases, testosterone increases, and aggression results.

Methodology and results
Resident intruder tests were used to measure the aggression and stress of male and female rats. In a colony of male and female rats a hierarchy is established, with male rats assuming a dominant role over the female rats. Different male and female rats were then introduced into the established colony. Male intruders were attacked 2.6 times and received 1.8 scars over 15 minutes. Female intruders were not the perpetuators or recipients of any attack.
Norepinephrine, dopamine and serotonin were measured by high performance liquid chromatography (HPLC) in various regions of the brain including hypothalamus (VMH), media amygdala (AME), lateral amygdala (ABL), and hippocampus (HPC). Norepinephrine content of VMH, AME, ABL, and HPC was not statistically different between the two sexes. However, values of dopamine in ABL of males were significantly less than corresponding female rats; levels of serotonin in the AME and ABL were also less in males compared to females.

Conclusion
In males, decreased serotonin in the amygdala was associated with increases in aggressive behaviour. Whether this relates only to the presence of the Y chromosome or to a combination of the Y chromosome and male hormone testosterone, remains to be determined.
Source: Authors Jonathon Toot et al , The University of Akron, Ohio presented at an American Psychological Society (APS) conference held October 17-20, 2001.