Individuals with defects in the MAOA gene that are associated with a low dopamine turnover rate have been shown to be more prone to aggressive behavior than their counterparts. The MAOA gene codes for the enzyme monoamine oxidase-A which plays a key role in the breakdown of neurotransmitters such as serotonin. Commonly known as the вЂwarriorвЂ™ gene, MAOA has a complicated heritage. In other words, no conclusions could be drawn about whether the gene caused a predisposition to substance abuse which then led to impulsive, crime-causing behavior or if it caused a predisposition to impulsivity which led to the cascade of substance abuse and criminal behavior - a problem involving what is known as the вЂњdirection of causalityвЂќ.ДЄnd then we also have the MAOA gene, our star-player.

But a lot of research on this topic is complicated since the researchers were unable to prove whether the functional variant of HTR2B, i.e a form of the gene that alters its function, was associated with substance abuse, or impulsivity. In 2010, Bevilacqua et al found that a codon in the HTR2B mRNA that signaled the termination of translation (also known as the stop codon) was associated with substance abuse and increased risk of committing impulsive crimes like homicide, arson, etc. For instance, the HTR2B gene encodes one of the several receptors for serotonin. Serotonin is a neurotransmitter that has been previously implicated in many functions including mood regulation and firing of the amygdala in the frontal lobe of the brain. Much of the genes involved in serotonin pathways, such as SCL6A4, have been studied for their implication in impulsivity and substance abuse. (1984) conducted a classic study showing the significant correlation between biological parents and adoptees for property crimes but not violent crimes. and how such effects were compounded by environmental factors such as substance abuse and childhood malnourishment. The landmark Italian sentencing came at a time when the social landscape was rife with studies linking genetics to a predisposition to violence, aggression, impulsivity, etc. However, at the appealвЂ™s court, the defense was allowed to conduct a вЂgenetic susceptibility testвЂ™ showing that the convict carried certain variants for the MAOA, COMT, SCL6A4 and DRD4 genes, that have been associated with aggression modulation, which later led to a modified sentencing from 9 years to 8 years.Д«ehavioral psychology, as a field, has always been intrigued by the etiology (the set of causes and manner of causation of a disease or condition) of violence. The convictвЂ™s sentence was reduced prior to this intervention because he had schizophrenia and was actively psychotic at the time of the crime. All the way back in 2010, an Italian judge made legal history by decreasing the already reduced sentence of a male convict accused of murder, after the convict was found to be a carrier of genetic variants that were thought to be associated with a predisposition for aggression (Forzano et al., 2010).

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