We already have genetic explanations for why blacks are more violent than other races.
For example - At one end of the androgen receptor gene, which is alternatively labeled AR or NR3C4, there are varying amounts of the trinucleotide repeats CAG and GGC. Blacks possess less of these CAG repeats.
Fewer CAG repeats causes increased transactivation of the receptor (meaning that the receptor became more activated to increase gene expression), see the research here:
Cortisol responses are another area that highlight racial differences in behavior. a new study by Way and Taylor has found that the short allele of 5-HTTLPR causes increased cortisol responses to a perceived social threat provided by the Trier Social Stress Test.
Also, a new study by Armbruster et al found that the 7R allele of DRD4 causes lower cortisol responses and that this allele interacted with the long allele of 5-HTTLPR to lower cortisol response to social stress.
Beaver et al determined that African-Americans have significantly more 7R alleles than whites. The possibility that low cortisol levels in African Americans could be partly genetic has important implications for research on the genetics of violence.
Indeed, in 2008 Rajender et al determined that male control subjects average 21.19 repeats, rapists average 18.44 repeats, murderers average 17.59 repeats, and men who murder after they finish raping average 17.31 repeats.
The shortest repeat lengths are associated with a higher degree of violence, earlier criminal records, verbal aggression, assertive personalities, extraversion, neuroticism, and self-transcendence (mystical tendencies).
Sjoberg et al proved that higher testosterone levels increased aggressive tendencies in males with the 3R allele that is the most common MAOA allele in black people but did not increase aggression in males with the 4R allele that is the most common MAOA allele in whites.
To summarize, African Americans have fewer CAG repeats in the androgen receptor gene, which somehow increases testosterone spikes. Higher testosterone decreases MAOA enzyme levels in those with the 3-repeat allele. Plus, androgen receptor alleles with fewer CAG repeats beget more androgen receptor activity. These receptors translocate to the cell nucleus and down-regulate the MAOA gene. African Americans likely have a significant genetic component to their lower baseline cortisol levels and lower cortisol spikes in response to threats. This further allows higher testosterone levels and decreases MAOA levels directly.