Hot weather could boost aggression — but only in certain conditions

Decades of research has shown that times of extreme heat are associated with both civil war and personal violence. Despite this consistent finding, there is controversy about why heat and violence go hand in hand. Is heat causing a cascading series of problems, such as crop failure, leading to civil unrest? Or does heat affect human decision-making directly?

A new study suggests that heat may make some people more aggressive, but only when they are already feeling marginalized. In most cases, however, heat doesn't directly affect people's decision-making.

"Given climate changes happening around the world, and changing temperatures, we felt this was an important line of inquiry," said Robert Pickmans, a doctoral student at the University of California, Berkeley, who co-authored the new research.

The findings have been released by the National Bureau of Economic Research as a working paper. Many past studies on the psychological effects of heat were done using small and limited sample sizes, Pickmans said. In the new research, the team recruited about 900 participants from Berkeley, California, and 1,000 from Nairobi, Kenya. The volunteers were taken to either a 71.6-degree-Fahrenheit (22 degrees Celsius) room or an 86 F (30 C) one, and were put through a standard battery of decision-making and cognitive tests. The researchers then compared the performance of the individuals in the cool and hot rooms.

The first finding was that, for the most part, there weren't a lot of differences. People in the hot rooms complained of feeling sleepier, but their decision-making abilities didn't slip.

"It looks like there is a degree of resilience," Pickmans told Live Science.

There was, however, an interesting result in one task, dubbed the "joy of destruction" task. In this test, participants got the opportunity to erase some of the savings of another participant. There was no major benefit or risk to doing so; they didn't get any monetary reward, but the other participant also wouldn't know how much this person chose to erase. Thus, Pickmans said, the task is a standard measure of aggression.

Heat didn't affect how people played the "joy of destruction" game in Berkeley, but in Nairobi, participants in the hotter room were more ruthless. Upon further investigation, the researchers discovered that this effect was driven by participants who were members of ethnic groups that had been marginalized in a contested election that was upending daily life in Kenya at the time.

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"We thought this was pretty interesting, especially given the climate literature that documents associations in temperature and political violence," Pickmans said. But he warned that the results should be taken as exploratory, because the researchers did not go into the study intending to look at differences between ethnic groups.

Also open to further research, Pickmans said, is whether temperatures above 86 F would yield more impacts on the human mind or behavior. He and his colleagues have been doing follow-up research in Berkeley on keeping people in 86 F rooms longer — two hours rather than one. So far, they don't see a huge erosion of ability, Pickmans said, though people do show some declines in certain kinds of reasoning and in overriding their gut impulses.

The findings suggest that researchers trying to predict the impact of climate change should focus on how climate will affect things such as resource availability, rather than on the heat itself affecting people's behavior.