Individuals with low levels of MAOA, it seemed, were predisposed to violence, and researchers came to refer to them as carriers of a “warrior gene.” Since the occurrence of this deficiency is tied to a defect in the X chromosome, men—possessing only one X chromosome, while women possess two—are more prone to the defect, although women may carry it and pass it on to their sons. Subsequent studies revealed that about one-third of the world’s male population carry the warrior gene, the expression of which can be triggered by childhood exposure to trauma.
