As temperatures rise, tempers flare. Anyone who has experienced the hostility of a swelteringly hot summer day in the city can attest to that.

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.

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