In 1978 behavioral scientist Han Brunner interviewed a patient at a Nijmegen hospital in the Netherlands whom was concerned about the extremely aggressive and violent behavior of the males in her family. This family had produce five generation of men that were attempted murders and rapists tracing back a hundred years to 1870. 
Since it was only men in this family that exhibited these antisocial and psychopathic tendencies, Brunner theorized that it was most likely an X-linked genetic mutation as the woman in the family have two X chromosomes that helps negate the faulty DNA. 
Through linkage analysis of the families DNA Dr. Brunner team had discovered the specific gene in fact existed on the short arm of the X chromosome. The mutated gene produced an inactive form of monoamine oxidase A that normally would help in breaking down neurotransmitters in the brain.  This can explain why antisocial behaviors in males is much more prevalent than in females. 
The mutation present in the Brunner studies are extremely rare as this is the only known case of a completely non-functioning MAOA. 
However, this discovery had lead to the identification of two other much more common alleles coding for this particular gene. These two alleles are MAOA-H (high-activity MAOA) and MAOA-L (low-activity MAOA). 
There is a clear correlation between physiological and chemical changes of the brain associated with males whom have the low activity MAOA gene. Since men with the MAOA-L alleles brain is bathed in excess dopamine and serotonin from birth the brain adapts by becoming less sensitive to these neurotransmitters thus decreasing activity in key areas of the brain that coordinate aggressiveness and abridging capacity for empathy.  
Furthermore, MAOA-L genotype has direct interaction with the size of brain structures. These individuals show a significant reduction of volume the entire bilateral amygdala with a maximum reduction in the anterior cingulate cortex. This resulted in an average decrease of 8% in brain capacity in these key areas when compared to the high activity MAOA gene.
Brain scans of these individuals reveal significantly reduced activity in the amygdala and ventral cingulate cortex when compared to MAOA-H patients. 
In humans, the amygdala is seen as a major contributor in moral reasoning and the emotions of fear . Impairment could lead to increased aggression and reductions in impulse control, morality, and/or empathy. 
These neurological changes have a clear and measurable tendency to create aggressive behavior when the subject is provoked becoming disproportionally angry than their high-activity counterparts when provoked because their brains are permanently altered with fewer serotonin receptors and neurons to cease the emotions of rage and anger. 
In one study, subjects took a quiz where they completed multiple choice questions and where rewarded points that could be converted for...