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Epigenetics are also likely to be important in the expression of behavioural illness through methylation and histone modification. These processes are heritable but can also be influenced by environmental factors such as smoking and abuse. Epigenetics may be one of the mechanisms through which the environment can impact the expression of the genome. Studies have also linked methylation of genes associated with psychopathic behaviour and nicotine and alcohol dependence in women, ADHD, and drug abuse.

A study found that children with the low activity MAO-A variant made them considerably more likely to develop behavioural problems if they had experienced some form of abuse or neglect. Children with the high activity MAO-A variant that experienced abuse or neglect also had a higher risk of behavioural problems. Children with the low activity MAO-A variant who did not experience abuse or neglect had about the same risk of developing behavioural problems as children with the high activity MAO-A gene.

A study found that participants with the low activity MAO-A gene, when faced with social exclusion or ostracism showed considerably higher levels of aggression towards those excluding them than other participants with the high activity MAO-A gene.

Another study also found that the low activity MAO-A could significantly predict aggressive behaviour in a high provocation situation, but was less associated with aggression in a low provocation situation.

A study by Gallardo-Pujol found that participants with the low activity MAO-A gene, when faced with social exclusion or ostracism showed considerably higher levels of aggression than other participants with the high activity MAO-A gene. Another study by McDermott found that the low activity MAO-A could significantly predict aggressive behaviour in a high provocation situation, but was less associated with aggression in a low provocation situation. Participants in the study were given a way to retaliate at another participant for stripping them of earnings made from an earlier part of the study. Participants with the low activity variant of the MAO-A gene were just as likely as participants with the high activity variant to retaliate when the loss was small. However, they were considerably more likely to retaliate and with greater force when the loss was large.

Epigenetics are likely to play an important role in the expression of the MAO-A gene through methylation and histone modification. Studies have also linked methylation of the MAO-A gene with nicotine and alcohol dependence in women. It is probable that epigenetic regulation as well as methylation profiling will play an increasingly important role in the study of the MAO-A gene and the way in which the environment affects its expression.