Scientists have long found evidence that heat affects the behavior of living beings. For example, American studies confirm that aggressive responses in rats toward members of their own species intensify at higher temperatures. In one experiment, 16 pairs of male rats were divided into two equal groups and subjected to 2.5 mA electric shocks at different frequencies: the first group received 12 shocks per minute, the second — 20. The procedure was repeated three times at different temperatures: 40°F (about 4.4°C), 70°F (about 21.1°C), and 100°F (about 37.7°C). It turned out that the rats' aggression was not tied to the shock frequency, but rather to the ambient temperature: the hotter it was, the more often the animals displayed aggressive behavior (such as trying to bite or scratch their opponent).
A similar trend has been observed in dogs, according to TASS. Research shows that heat triggers aggressive behavior in dogs toward humans. When researchers from Harvard analyzed data on 69,525 dog bite incidents in eight U.S. cities from 2009 to 2018, they found that dogs were more likely to attack people in hot weather. According to their calculations, the relative risk of being bitten by a dog at 0°F (about –17.7°C) was below one, while at 90°F (roughly 32°C), it was above 1.2. On average, experts explain, the number of dog bites increases by 4% in higher temperatures.
Even fish become more aggressive in the heat: scientists discovered that Nile tilapia (Oreochromis niloticus) engage in more active dominance fights at a water temperature of 26°C than at 14°C.
However, it’s not just instinct-driven animals that feel the impact of heat — humans do, too. To begin with, the human body is not adapted to extreme heat: our internal cooling mechanism — sweating — becomes ineffective when the air is too hot and humid. This leads to heatstroke, which can result in multi-organ failure (kidneys, heart, and brain), possibly causing disability or even death. According to experts, a temperature of 35°C combined with 100% humidity is considered fatal. Under such conditions, sweat no longer evaporates and the body cannot cool down, meaning even a well-prepared person with unlimited access to water may die within a few hours. But physical health isn’t the only danger posed by extreme heat.
One historical event that prompted scientific interest in the link between heat and violence was the outbreak of race-related riots across U.S. cities during the summer of 1967. Weather-wise, it was one of the hottest summers in American history — in Coeur d'Alene, Idaho, for example, temperatures hovered around 37°C for a record 16 days. During that three-month period, approximately 160 riots were reported nationwide, the deadliest resulting in 43 fatalities. The media dubbed it "The Long, Hot Summer," reflecting the widespread belief that heat and violence are closely connected.
That theory was later backed by science. A 1997 U.S. study analyzed the relationship between average annual temperatures and crime rates from 1950 to 1995. It found that in years with more days reaching 32°C or higher, there were more violent crimes and assaults. Researchers also observed that high temperatures correlated with an increase in terrorist attacks, related deaths, and mass shootings. A 2023 study revealed similar findings: sudden temperature spikes (especially in winter) led to a rise in violent crimes in U.S. cities. On days of sudden warming, robberies increased by 4%, aggravated assaults by 9.4%, and homicides by 19.4% compared to the annual average.
It is believed that such troubling statistics are partly due to people becoming more aggressive in the heat. For example, studies by American psychologist Craig Anderson showed that people are more likely to interpret others’ actions as aggressive when they’re hot. To test this, he showed students four video clips of a couple talking: the first video portrayed a neutral interaction, while the others showed escalating tension. Participants were asked to rate how hostile the people in the videos seemed. Meanwhile, the students were placed in rooms at temperatures ranging from 14°C to 36°C. Those in the hotter rooms rated even the neutral clip as aggressive.
Moreover, some studies suggest that heat directly drives people toward violent behavior. For instance, an analysis of inmate behavior in a Mississippi prison showed that on hot days (when temperatures reached around 80°F or 26.6°C), incidents of violent interaction among prisoners increased by 16%.
This behavior is likely due to increased levels of cortisol, adrenaline, and testosterone — hormones associated with dominant, aggressive behavior — in the bloodstream during hot weather. However, more experts now believe that heat mainly acts as a trigger that releases previously accumulated stress. In one experiment, participants played computer games in rooms with different temperatures. The scientists aimed to prove that players would express more aggression toward their opponents in hotter environments. But the results indicated that the aggressive behavior could also be linked to social factors, not just temperature.
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