But researchers are now quantifying the causal relationship between extreme climate and human conflict. Whether their focus is on small-scale interpersonal aggression or large-scale political instability, low-income or high-income societies, the year 10,000 B.C. or the present day, the overall conclusion is the same: episodes of extreme climate make people more violent toward one another.
In a paper published this month in the journal Science, we assembled 60 of the best studies on this topic from fields as diverse as archaeology, criminology, economics, geography, history, political science and psychology. Typically, these were studies that compared, in a given population, levels of violence during periods of normal climate with levels of violence during periods of extreme climate. We then combined the results from those studies that concerned modern data in a “meta-analysis,” a powerful statistical procedure that allowed us to compare and aggregate findings across the individual studies.