One factor which explains part of why Blacks have higher than average crime rates is testosterone. Testosterone is known to cause aggression, and Blacks are known to at once have more of it and, for genetic reasons, to be more sensitive to its effects.
Testosterone activity has been linked many times to aggression and crime. Meta-analyses show that testosterone is correlated with aggression among humans and non human animals (Book, Starzyk, and Quinsey, 2001). Women who suffer from a disease known as congenital adrenal hyperplasia are exposed to abnormally high amounts of testosterone and are abnormally aggressive. Artificially increasing the amount of testosterone in a person’s blood has been shown to lead to increases in their level of aggression (Burnham 2007; Kouri et al. 1995). In fact, a study of Rhehus monkeys found that injecting female fetuses with testosterone caused them to behave just as aggressively as young males (Book, Starzyk, and Quinsey, 2001). Finally, people in prison have higher than average rates of testosterone (Dabbs et al., 2005). Thus, testosterone seems to cause both aggression and crime.
Richard et al. (2014) meta-analyzed data from 14 separate studies and found that Blacks have higher levels of free floating testosterone in their blood than Whites do.
Exacerbating this problem even further is the fact that Blacks are more likely than Whites to have low repeat versions of the androgen receptor gene. The androgen reception (AR) gene codes for a receptor by the same name which reacts to androgenic hormones such as testosterone. This receptor is a key part of the mechanism by which testosterone has its effects throughout the body and brain.
All versions of this gene have several repeats of a CAG nucleotide sequence. Variants between people differ in the number of these repeats. Versions of this gene with fewer than average numbers of repeats lead to increased androgen activity and has been associated many times with crime and aggression. For instance:
Rajender et al. (2008) analyzed data on 645 men including 241 convicted rapists, 107 convicted murders, and 26 men convicted for both rape and murder. They found that those incarcerated for both rape and murder had the shortest average number of CAG repeats followed by murderers, followed by rapists. The non criminal controls had the longest CAG repeats.
Butovskaya et al. (2013) conducted a study in Tanzania in which an association between anger, aggression, and AR genotype was found among 138 adult men.
Commings et al. (2003) found an association between AR genotype and the probability that a person’s parents were divorced and that their father was present in the first 7 years of life. They also found the expected correlations between AR genotype and agression, but, possibly due to small sample size, they were not statistically significant
Similarly, Pichard et al. (2007) found that AR genotype predicted anti-social personality in a sample of 1,007 men
Hurd et al. (2010) found a significant correlation between AR genotype an anger and physical aggression, and insignificant but positive relationships with verbal aggression and hostility in a sample of 188 men.
Jonsson et al. (2001) found non-significant relationships with aggression and impulsivity in a sample of 335 Swedes.
Cheng et al. (2006) study found that violent criminals were more likely than controls to have short CAG repeats. However, the difference in means, while in the expected direction, was not statistically significant.
What we see in this literature then is a mix of positive and significant effects and positive and insignificant effects for the relationship between shorter AR genes and aggression/crime. This distribution of effect sizes strongly suggests that the true mean effect is real and positive. On a theoretical level, this finding makes a great deal of sense in light of the role of testosterone in aggression and crime and what we know about the function of the AR gene.
Four separate studies have shown that low repeat alleles of the AR gene are more common among Blacks than among Whites (Irvine et al 1995, Wang et al. 2013, Bennet et al. 2002, and Shibalev et al. 2013). Given this, it is highly likely that low repeat AR alleles increase Black crime rates by increasing the impact of testosterone. Due to testosterone’s impact on sexuality, these same mechanisms may also explain why Blacks have such high rate of single parenthood. This is especially likely in light of the fact that, as seen above, low repeat AR alleles predict family structure.