Heat Makes People Miserable but Not Selfish, a Five-Country Study Finds

Heat and violence have a well-documented statistical relationship. Hotter temperatures correlate with more assaults, more civil conflict, and more aggressive behavior at the population level. It is one of the most consistent findings in climate-behavior research, and in a warming world, it carries alarming implications.

But correlation is not mechanism. Does heat directly make individuals less cooperative and more hostile? Or does it operate indirectly, through crop failures, economic stress, disrupted sleep, and increased alcohol consumption that then fuel conflict?

A new study by Alessandra Cassar of the University of San Francisco and colleagues tested the direct path. They put people in hot rooms and watched what happened to their economic behavior. The answer: nothing much.

The experimental design

The researchers recruited university students in five countries: Colombia, India, Kenya, Mexico, and the United States. Participants played a series of economic games designed to measure specific prosocial and antisocial behaviors: egalitarianism (caring about equal outcomes), resource maximization (caring about the total payoff), selfishness, spite (reducing another player's payoff at a cost to yourself), and competitiveness.

The key variable was room temperature. Some participants played in rooms cooled to 18 degrees Celsius. Others played in rooms heated to as high as 34 degrees Celsius. To test whether stress interacted with heat to produce antisocial behavior, some players were first made to lose a competitive game before playing the economic games.

Miserable but fair

At temperatures above 30 degrees Celsius, participants reported significantly more frustration, tiredness, and unhappiness. The heat was genuinely unpleasant. But neither heat nor the stress of losing had any systematic effect on the economic behaviors measured. People in hot rooms were no more selfish, no less egalitarian, no more spiteful, and no more competitive than people in cool rooms.

The finding held across all five countries, despite significant cultural differences in baseline prosociality. Participants from the United States, for instance, cared less than those in other countries about equal outcomes between players but cared more about maximizing the total payoff, even if it was unevenly distributed. These cross-cultural patterns were robust, but temperature did not shift any of them.

Gender mattered more than temperature

The strongest predictor of prosocial behavior in the study was not heat or stress but gender. Women behaved in more egalitarian ways than men in every country studied. Women also demonstrated less competitiveness than men in every country except Kenya. These gender effects were consistent and substantial, dwarfing any potential temperature effect.

What this means for the heat-violence link

If heat does not directly alter individual economic behavior or cooperation, the well-documented relationship between heat and societal violence likely operates through indirect channels. Heat damages crops, strains infrastructure, disrupts livelihoods, increases alcohol consumption, and degrades sleep quality. Each of these intermediaries can erode social stability and increase conflict without any individual person becoming inherently less cooperative.

This distinction matters for policy. If heat-driven violence is mediated by economic stress and institutional strain rather than by a direct psychological effect on individuals, then the solutions lie in climate adaptation, agricultural resilience, and social safety nets, not in attempting to modify individual behavior during heat events.

Important limitations

The study used university students, a population that is younger, more educated, and generally healthier than the broader population in any of these countries. Whether the findings extend to older adults, people with chronic health conditions, or populations under existing economic stress is unknown.

The heat exposure was brief. Participants spent a limited time in temperature-controlled rooms. The cumulative effects of days or weeks of extreme heat, with disrupted sleep, dehydration, and compounding irritability, could produce different results than a single controlled session.

Economic games, while useful for measuring specific behavioral dimensions, are abstractions. The stakes are small, and the social context is artificial. Whether the same null result would hold in real-world situations involving genuine resources, personal relationships, or institutional power is a separate question.

The temperature range tested, up to 34 degrees Celsius, is unpleasant but not extreme by the standards of many tropical and subtropical regions. Temperatures regularly exceed 40 degrees Celsius in parts of India, Mexico, and East Africa. Whether more extreme heat might cross a threshold that moderate heat does not remains untested.

Despite these caveats, the study provides clear evidence against one popular assumption: that heat, by itself, makes people behave worse toward each other. The reality appears to be more complicated, and more addressable, than that.