Randy Thornhill, an evolutionary psychologist at the University of New Mexico in Albuquerque, has spent years amassing evidence for his "parasite stress" model of human society, which considers all disease to be a parasite on human society. He has already used it to predict that people in disease-ridden regions will be more xenophobic, and prefer to associate with relatives and close neighbours. These "collectivist" societies opt for strongly conservative values and autocratic governments, which Thornhill says minimises the risk of contracting diseases. By contrast, people in countries with low disease rates tend to be more individualistic and democratic, he says.With Corey Fincher, also at the University of New Mexico, Thornhill has now found a link between disease and violence. The pair compared murder and disease rates from 48 US states and found that high disease rates correlated with high murder rates. The pattern held even when they took into account economic inequality within the society, which also increases the murder rate(Philosophical Transactions of the Royal Society B, DOI: 10.1098/rstb.2011.0052).
"If you clean up the diseases you'll reduce the rates of homicide," he [Thornhill] says. He predicts that reducing disease rates should cut the murder rate within 20 years as a new generation grows up in a healthier environment.
To be sure, correlation is not causation, so we have to be careful not to jump to quick conclusions. There are a lot of things going on in "disease-ridden" societies that may increase the level of violence. On the other hand, there can hardly be an objection to reducing disease rates in any society and while we don't need the promise of lower murder rates as an incentive to lower disease rates, it certainly would be nice if things really worked that way.