“The heat made people crazy. They woke from their damp bed sheets and went in search of a glass of water, surprised to find that when their vision cleared, they were holding instead the gun they kept hidden in the bookcase.’’

This passage, from Summer Island, a romance novel by Kristin Hannah, is how researchers introduce a potentially important new study they believe could alter peoples’ attitudes about the impact of unrelenting heat on violence, and why some parts of the world experience strikingly higher rates of violence than others.

It’s not what people think. The new research goes beyond existing ideas about how hot summer nights cause tempers to flare and prompt sporadic acts of violence. Their model explores long-term cultural changes resulting from persistently high temperatures and a lack of seasonal variability, among them a loss of self-control and future-oriented goals. This combination can lead to more aggression and violence, they say.

“People think about weather when they think about global warming, but don’t realize that climate change can increase aggression and violence, ’’ says Brad Bushman, professor of communication and psychology at The Ohio State University and one of the study’s authors. “But climate change affects how we relate to other people.’’ Moreover, he predicts that unmitigated global warming could increase violence levels in the United States, something he believes deserves immediate attention.

Bushman, with colleagues Paul Van Lange, professor of psychology at Vrije University in Amsterdam (VU) and research assistant Maria Rinderu, also of VU, say their model, which they call CLASH (for CLimate Aggression and Self-Control in Humans), recently published in the journal Behavioral and Brain Sciences, could explain why violence is greater in countries closer to the Equator and in the Southern regions of the United States, and less so in the American North and in areas farther away from the Equator.

People living in such climates are more tuned into the present — the here and now — and are less likely to plan for the future, they theorize. They are less strict about time, less stringent about birth control, and have children earlier and more often, Bushman says.

“If you live farther away from the Equator, you have to exercise more self-control,’’ Bushman says. “You can’t just eat all your crops, because you then won’t have anything left to eat in the winter. But if you live closer to the Equator, those mango trees will grow mangoes year-round.’’

This scenario encourages a state of mind and lack of self-control that affects how people treat each other, according to Bushman. “Climate shapes how people live, and affects the culture in ways that we don’t think about in our daily lives.’’ Such a faster life strategy “can lead people to react more quickly with aggression and sometimes violence,’’ Bushman adds.

A model of CLimate, Aggression, and Self-control in Humans (CLASH) Source: Lange, Rinderu and Bushman, 2016

Until recently only two models helped explain why violence and aggression are higher in hotter climates. The first, the General Aggression Model — which Bushman helped develop — holds that hot temperatures make people feel uncomfortable and irritated, causing them to become more aggressive.

The second, known as the Routine Activity Theory, suggests that people go outside and interact with each other more when the weather is warm, thus providing more opportunities for conflict. But that doesn’t explain why there is more violence when the temperature is 95 degrees F (35 degrees C) than when it is 75 degrees F (24 degrees C), even though people are more likely to go outside under both conditions.

To be sure, “our ability to cope with irritation and frustration may be less strong on hot days,’’ says Van Lange, the study’s lead author. “But this would be only part of the story. We thought it is not only average temperature that might matter, but also seasonal variation in temperature. The latter is predictable and may lead cultures that are facing seasonal variation to develop stronger norms and habits, and adopt longer-time planning and self-control — that is, to forgo immediate benefit for longer-term benefit.’’

These two factors, average temperature and predictable seasonal variation, may help experts better understand aggression, as “the psych literature has revealed that self-control is one of the strongest predictors of aggression and violence,’’ Van Lange adds.

It also may explain why crime is higher in the American South, Bushman says. “Violent crime rates have always been higher in the South,’’ he says. “You see different life strategies in the North and the South. People seem to plan more for the future in the North. But we predict that if climate change continues, with less seasonal variability in the North, you will see violent crime rates increase there, too.’’

What about climate’s influence on war? “War is usually less impulsive, less the result of lack of self-control, and more planned and premeditated,’’ Bushman says. “However, the model could be applicable to a leader inclined to respond impulsively,’’ he says.

The scientists have called for more research, and note that they are not suggesting people in hotter climates can’t help themselves when it comes to violence. However, they stress that it is important to recognize that culture is strongly affected by climate.

“Climate doesn’t make a person, but it is one part of what influences each of us,’’ Van Lange says.

Marlene Cimons writes for NexusMedia, a syndicated newswire covering climate, energy, politics, art and culture.