A couple of years ago Solomon Hsiang, assistant professor of public policy at University of California, Berkeley, became fascinated by an important issue: what effect does heat have on our brains – and emotions? It is a pertinent question right now, given that temperatures have soared this summer across most of Europe and the US – currently, some 70 million Americans are still under some level of “heat advisory” notice. 

Some readers will have welcomed the warmer weather. If you live in Scotland or Sweden, where you don’t often get the chance to spend time on the beach in August, summer sunshine has historically been associated with joy. Conversely, if you live in a city with minimal air conditioning (London, for example) or in one of those sweltering US states, a sharp rise in temperature is greeted with horror. 

But what Hsiang and his research team wanted to know is whether there is something about hot weather that destabilises our brain and makes us violent. This is not a new issue: a recent meta-analysis of 60 prior studies showed that unstable temperatures tend to be correlated with conflict.