High temperatures have long been empirically linked to violence, conflict, and aggression at the societal level—a troubling pattern in a warming world. Alessandra Cassar and colleagues sought to explore the effect of high heat on individual egalitarianism, resource maximization, selfishness, spite, and competitiveness. The authors invited university students in Colombia, India, Kenya, Mexico, and the United States to play games that involved making choices about whether to share, whether to reduce another player’s payoff at a cost to oneself, as well as whether or not to compete. Some players were first stressed by losing a competitive game. The key variable was that some players played in rooms where the heat was as cool 18°C, while others played in rooms as hot as 34°C. At temperatures above 30°C, participants reported significantly more frustration, tiredness, and unhappiness, but neither heat not the stress of losing had any systematic effects on egalitarianism, resource maximization, selfishness, spite, or competitiveness. While the results suggest that heat, per se, does not significantly influence economic decisions or prosocial behavior, there were clear links between gender and prosociality, with women behaving in more egalitarian ways than men in every country and demonstrating less competitiveness than men in every country except Kenya. There were also cultural differences between countries in specific elements of prosociality. Participants from the United States, for example, cared less than the other countries that outcomes were equal between players—but cared more than average that the total summed payout to both players was as high as possible, even if it was unevenly distributed. According to the authors, while heat can have serious indirect effects—damaging crops, straining societies, and increasing alcohol consumption in ways that may fuel violence—the authors find no evidence that heat alone makes people more selfish or less cooperative.
PNAS Nexus
Gender and culture shape prosociality more than heat stress in a five-country experiment
10-Mar-2026