In The 10,000 Year Explosion, Cochran and Harpending (2009) state:
“For example, there is a gene whose ancestral form helps people to conserve salt. Since humans spent most of their history in hot climates, this variant was generally useful. A high frequency of this ancestral allele among African Americans probably plays a role in their increased risk of high blood pressure today. In tropical Africa, in fact, almost everyone has the ancestral version of the gene. In Eurasia, a null variant (one that does nothing at all) becomes more and more common as one moves north.4 Perhaps the gene’s action of promoting salt conservation becomes harmful —by causing higher blood pressure—in cooler areas, where people sweat less and lose less salt.”
People of African ancestry are more likely to carry the positive allele which allows them to conserve salt, and as a consequence, face higher blood pressure. When people spread from Africa, Europeans and Asians became disadvantaged by the allele as they were in much colder temperatures, and the allele then became neutral. A similar example is one of the explanations for why Africans lost the sickle-cell anemia allele when they were transported to North America – it only became negative and therefore was evolutionarily maladaptive.
The study Cochran and Harpending cites is Thompson et al. (2004), who state in the Discussion:
“Phenotypic variability in human hypertension susceptibility could be due to genetic variation in genes that underlie salt regulation. The “sodium retention hypothesis” (Gleibermann 1973; Nakajima et al. 2004) proposes that ancient human populations living in hot, humid areas with low sodium availability adapted to their environment by retaining salt, whereas populations in cooler, temperate climates adapted to conditions of greater sodium availability. Since an increased prevalence of hypertension in African American versus white populations is well documented and since blood pressure homeostasis is strongly influenced by sodium regulation and salt sensitivity, population-specific differences in susceptibility to hypertension could be explained, in part, by variation in genes related to salt regulation. This hypothesis provides a general framework for interpreting the patterns of variation observed at genes involved in sodium homeostasis.”
Below is a chart from the CDC showing blood pressure by country:
Clearly, other factors would be at play here, which no one denies. Hence, why you have countries such as Russia being comparable to Africa. Other evidence supports a racial admixture hypothesis. For example, Harburg et al. (1973) finds that darker skin color within races and between races is associated with higher blood pressure and higher stress levels. This was replicated by Harburg et al., (1978).
Cottington et al., (1985) finds that even when filtering out people who are diagnosed with high blood pressure, blood pressure was still positively associated with higher levels of aggression. Similar results were found by Jorgensen et al., (1996), though they argue this is partially mediated by multiple variables. Some of these variables fall victim to the sociologist’s fallacy, though. Coulson et al., 2016 was unable to find blood pressure and crime were associated at the individual level, but they did find an association at the group level and that higher blood pressure correlated with poverty as well.
Like everything, more research needed; specifically I would like to see the CYP3A5*1/*3 polymorphism (related to higher sodium conservation) put in regression with violent behavior and aggression in individuals., controlling for race, sex, etc.