Disease-ridden societies could be more murderous

DOES the threat of rampant disease leave people more likely to commit murder? It's a provocative suggestion, that, if correct, should provide even more incentive to improve the quality of public healthcare in countries where disease is rife.

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).

The idea tallies with what we know about different countries' murder rates, says Martin Daly of McMaster University in Hamilton, Canada. A recent study identified a link between collectivist societies and murder rates, but did not look at disease rates (Homicide Studies, DOI: 10.1177/1088767911406397).

"Thornhill has pretty convincingly established a link between parasite stress and violence," says Carlos David Navarrete of Michigan State University in East Lansing.

Others are not yet ready to accept the link, though. "It's fascinating and I'd like it to be true," says Val Curtis of the London School of Hygiene and Tropical Medicine, but she points out that there may be other factors at work. For instance, although the research takes into account relative economic inequalities within the society, it does not consider absolute wealth. Poverty itself may lead to higher murder rates - but because poor societies are likely to have relatively weak healthcare systems and higher levels of disease, there might still be a strong correlation between disease and murder.

If Thornhill's hypothesis is right, it should be possible to see a change in the murder rate as a society faces a reduced or heightened disease risk even as the levels of wealth in the society remained constant. The US data was not detailed enough to allow such an analysis. He predicts that simply investing in healthcare - but not necessarily any other aspect of society - could have an effect on the murder rates.

"If you clean up the diseases you'll reduce the rates of homicide," he 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.

"I'm not sure about that," says John Archer of the University of Central Lancashire in Preston, UK. He says social systems can linger for decades, even if the original cause disappears.

"The place to try this out is Africa," Curtis says. There are many projects under way to improve public health in disease hotspots, and it would be simple to track any effects on violence, she says.