Do you know if T sensitivity has been examined for differences by race? For example, imagine that blacks produced twice as much of the T receptor. This should get them somewhat more masculinization at the same concentration of T.What I think is undeniable is that "masculinization" is not simply a function of circulating testosterone levels and Rushton's notion of testosterone as a "master switch" that neatly explains myriad racial differences is overly simplistic at best.
Racial variation exists in the Androgen Receptor gene, but there's no reason to assume this variation underlies global differences in "T sensitivity" between races, or that racial differences in "masculinization" in a given domain should imply similar racial differences in masculinization in every other domains. Ultimately, the androgen/AR pathway signals/regulates genes, which themselves vary between populations. Without knowing what's happening downstream of the AR, racial differences in the AR gene are not terribly informative.
To give a concrete example of variation in the AR gene failing to predict a racial difference in a phenotype:
The most-studied polymorphism on the Androgen Receptor gene is a CAG repeat found to be "inversely correlated with transcriptional activity by the androgen receptor". Blacks average fewer repeats than whites, implying increased transcription by the AR in blacks. Intra-racially, a lower number of CAG repeats is associated with higher sperm concentrations. If those facts led you to predict black men have higher sperm counts than white men, you predicted wrong.
Could the differences in sperm concentration in various geographical areas be related to the ethnic origin of the studied populations? From the results shown in Table 3 it seems that the MSC of healthy men might be lower in Africa and South East Asia compared to other parts of the world.I'm unable to find direct comparisons between white Americans and American blacks, but the evidence I'm aware of is consistent with lower sperm counts in the latter: autopsy weighing indicates American blacks have smaller testes than whites. Another autopsy study:
[Semen quality and male reproductive health: the controversy about human sperm concentration decline]
The mean sperm density among the fertile population in this series was 71.2 million/ml which is statistically greater than the value of 46.8 million/ ml observed in the infertile marriage population (P<0.001). The mean sperm count reported in the literature for the Caucasian fertile population is higher, with a range between 79-137 million/ ml [Seminal analysis in fertile and infertile Nigerian men]
For these preliminary studies, testes from five non-Hispanic Caucasian, five Hispanic, and five African American males of ages 28.2 ± 1.3, 27.8 ± 1.6, and 28.2 ± 1.3 years, respectively, were evaluated. Based on this small sample of men, there were no statistical differences among groups, respectively, for paired parenchymal weight (44.8 ± 5.3, 40.7 ± 1.8, and 34.8 ± 4.6 g), for daily sperm production per gram of testicular parenchyma (4.9 ± 0.3, 5.1 ± 0.7, and 4.3 ± 0.5 X 106/g), or for daily sperm production per man (224 ± 36, 211 ± 35, and 157 ± 33 X 106).These findings are not statistically significant due to the very small sample sizes, but they are consistent with the other evidence.