One gene which plays a role in Black’s high crime rate is the monoamine oxidase -a gene (MAO-A). This gene produces an enzyme by the same name. The enzyme MAO-A breaks down a class of neurotransmitter called mono-amines in the brain. These neurotransmitters include ones which are well known to effect behavior such as dopamine and serotonin. Some versions of the MAO-A gene lead to lower levels of MAO-A the enzyme and, therefore, more mono-amine activity in the brain.
The molecular difference between different versions of the MAO-A gene lies in the number of times a certain section for its promoter region is repeated. Three repeat alleles lead to lower than average levels of the enzyme MAO-A. The two repeat allele leads to even lower levels of MAO-A.
Beginning in the 1990’s, researchers began to suspect that variation in MAO-A levels might be related to traits like aggression and criminal behavior. This suspicion arose following a study on the particular family which carried a rare mutation that lead to extremely low levels of the enzyme MAO-A. This family had rapists, arsonists, and other violent criminals (Brunner et al. 1993).
Later, geneticists raised rates with an artificial version of the MAO-A gene which lead to low MAO-A production. It was found to cause the rats to act far more aggressive than is typical (Cases et al. 1995, Scott et al. 2008, Vishnivetskaya et al. 2007, and Mejia 2002).
The first direct link between common MAO-A alleles and violence in humans came from Caspi et al 2002. This study found that people with either the 2R or 3R version of the MAO-A gene tended to be more aggressive than average, but only if they had difficult upbringings. Based on these results, it was hypothesized that low activity MAO-A genes might cause people to have stronger reactions to negative experiences than people with high activity versions of the gene. Since then, a meta-analyses has confirmed that this interaction between MAO-A genotype and childhood trauma has been well replicated in male subjects (Bryd and Manuch 2013).
Source: Caspi et al. 2002
Since then, researchers have found that low activity MAO-A also predict higher levels of anti-social personality even among those who have not experienced child abuse (Ficks and Waldman 2014). These versions of the gene have also been linked to high levels of credit card debt and obesity (Camarena et al. 2004, Neve and Flower 2009, Need et al. 2005, and Feummeler 2008). Low activity versions of the gene have also been shown to significantly increase the risk that someone will end up in prison (Roux 2014, Vaughn et al. 2009, Beaver et al. 2009, and Beaver et al. 2013). Given this, it seems plausible that low activity versions of the MAO-A gene leads to increased impulsivity, likely due to increased dompanie activity, which in turn leads to an elevated risk for criminal activity, aggression, obesity, and credit card debt.
Racial differences in MAO-A genotypes are well established. Below you can see a review of the literature in the United States. The vast majority of the research shows that Blacks are more likely than Whites to carry both versions of the low repeat allele.
|Study||Sample Size||Sample Type||Sex||Black 2R %||White 2R %|
|Beaver et al. (2013)||2574 Whites164 Blacks||Nationally Representative Sample (ADD Health)||Male||5.2||0.1|
|Widom and Brzustowicz (2006)||180 Whites98 Non-Whites||Abuse victims||Male||6.12||0.6|
|Widom and Brzustowicz (2006)||196 Whites143 Non Whites||Abuse Victims||Female||2.6||7|
|Reti et al. (2011)||365 Whites232 Blacks||Psychiatric Patients||Both||4.7||.5|
|Choe et al. (2014)||106 Whites86 Blacks||N/A||Male||6.0||0|
|Study||Sample Size||Sample Type||Sex||Black 3R%||White 3R%|
|Beaver et al. (2013)||2574 Whites164 Blacks||Nationally representative sample (Add Health)||Male||52.3||37|
|Widom and Brzustowicz (2006)||180 Whites98 Non-Whites||Abuse victims||Male||35.7||41.1|
|Widom and Brzustowicz (2006)||196 Whites143 Non-Whites||Abuse victims||Female||74.1||62.8|
|Rosenberg et al. (2006)||274 Whites37 Blacks||N/A||Male||54.1||36.1|
|Reti et al. (2011)||365 Whites232 Blacks||Psychiatric patients||Both||48.7||34.1|
|Choe et al. (2014)||106 Whites83 Blacks||Low income||Male||42.2||30|
|Sabol, Hu, and Hamer, (1998)||539 Whites52 Blacks||Various combined samples||Both||59.1||33.1|
In summary, Blacks have higher levels of low repeat MAOA alleles than Whites do, this explains part, though not all, of why Blacks have higher crime rates than Whites.