Epigenetics and MAOA in the brain
Epigenetics is revolutionizing how scientists think about genetics. Epigenetics refers to external changes to DNA that turn genes “on” or “off” without altering the DNA sequence [26]. Gene expression — the manifestation of genetic potential — is modified in epigenetic processes, even though the gene itself stays intact. The field of epigenetics is largely theoretical, at least insofar as humans are concerned. But growing evidence suggests that epigenetic changes can, in some cases, be passed on from parents to children. They are handed down not as inherited traits, but as non-hereditary modifications transmitted to offspring along with genes from their parents [26].
Various environmental factors are thought to influence epigenetic processes. Could epigenetics modify behavioral traits by acting on MAOA gene activity? Scientists are just beginning to understand the effects of MAOA variants on the brain. The low-expression MAOA-3R variant has been linked with a heightened response from the amygdala, a structure in the brain that regulates emotion [27]. 3R is also associated with decreased activity in prefrontal regions of the brain that protect against anxiety [27].
Elena Shumay of the Brookhaven National Laboratory and her team conducted a study to determine how MAOA variants affect brain levels of the MAOA enzyme in healthy men [28]. Using PET imaging scans, these researchers found no correlation between MAOA brain levels and MAOA gene variants. Shumay and her colleagues reasoned that MAOA levels must be regulated by the same region of the MAOA gene where the 2R, 3R, 4R, or other repeat sequence are located. The evidence supported their prediction: it appears that MAOA expression associated with MAOA brain levels is under the control of epigenetic mechanisms [28].
In other words, epigenetics may influence whether a tendency toward higher or lower MAOA genetic activity actually manifests itself. The amount of genetic activity, in turn, determines whether there is a larger or smaller quantity of the MAOA enzyme in the brain, which is needed to break down certain neurotransmitters [28].

There are limits to studying the role of a single gene in antisocial behavior outside of its environmental context. Even when a gene correlates closely with violence or criminal acts, it does not mean that the gene itself codes for aggressive tendencies. According to Kevin Beaver and University of California at Davis’ Jay Belsky, plasticity genes seem to affect how much or how little male youth are influenced by their parents. Beaver and Belsky claim these genes appear to increase susceptibility to environmental effects, “for better and for worse” [29]. Supportive and unsupportive parents are more likely to have a positive or negative impact, respectively, on their children if their kids carry plasticity genes [29].
Yet plasticity genes appear to have a cumulative effect.