In 1993, after fifteen years of investigation, researchers identified a deficiency in a gene that produces an enzyme called monoamine oxidase A, or MAOA, a key regulator of impulse control. 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
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