A new model attempts to explain the higher violent crime rates seen in parts of the world near the equator with hotter climates. It centers on the belief that hot weather without much seasonal change leads to less self-control, contributing to more aggression and violence.

A team based at a university in the Netherlands, Vrije Universiteit Amsterdam, developed the CLASH model, for CLimate Aggression, and Self-control in Humans.

"Climate shapes how people live, it affects the culture in ways that we don't think about in our daily lives," said lead researcher Paul Van Lange. "We believe our model can help explain the impact of climate on rates of violence in different parts of the world."

The new model attempts to build upon earlier explanations for the higher levels of violence and aggression seen in hot climates, which posit that higher temperatures lead to more general discomfort and irritation, and that because people are outside more, there are more opportunities for interaction and conflict.

The CLASH model suggests that it isn't only the heat that contributes to violence, but also the lack of seasonal variation in the weather closer to the equator.

"Planning in agriculture, hoarding, or simply preparing for cold winters shapes the culture in many ways, often with people not even noticing it. But it does shape how much a culture values time and self-control, " Van Lange said. "If there is less variation, you're freer to do what you want now, because you're not preparing foods or chopping firewood or making winter clothes to get you through the winter. You also may be more concerned with the immediate stress that comes along with parasites and other risks of hot climates, such as venomous animals."

The researchers say this leads to what is called a "fast life strategy" and a focus on the present moment that requires less self-control in general, something that could lead people to react more quickly with aggression and even violence.

The team is explicit in clarifying that the theory shouldn't be taken to suggest people in hotter, equatorial climates can't help but be violent and aggressive.

"How people approach life is a part of culture and culture is strongly affected by climate," Van Lange said, adding that more research must be done to see if the theory is correct. "Climate doesn't make a person, but it is one part of what influences each of us. We believe it shapes the culture in important ways."

The CLASH model is outlined in an online article in the journal Behavioral and Brain Sciences.

The paper acknowledges there are exceptions to the rule, such as South Africa, which is further from the equator but has a particularly high violent crime rate. We might also point to locales like Singapore and northern Australia that sit near the equator where violent crime is relatively rare.

It could be argued that the correlation between violence and other factors like economics or history actually appears stronger than the link to heat and lack of seasonal variation. But the researchers argue that climate drives these factors as well.

"Differences between countries (e.g., historical, economic, political variables) are exceptionally difficult to disentangle from climate differences," reads the pre-print version of the paper, which goes on to suggest that "over time cultures have evolved such that economic growth and prosperity is smaller to the degree that one is closer to the equator. Also, this reasoning may help explain the existence of what has been described as the equatorial grand canyon, the hot belt several thousand kilometers around the equator, characterized by an exceptionally large concentration of lower-income countries. Thus, violence and poverty often operate in concert because they are both rooted in traits such as fast life strategy, an orientation on the present, and little self-control."

The research also looks at comparisons within large countries like the United States, and points out that violent crime rates are significantly higher in warmer southern states than in the north.

"Reserving judgment and giving benefit of the doubt is probably an effective mindset, because provocation may be more quickly elicited in individuals from Southern states than in individuals from Northern states, and, once elicited, more quickly translate into aggression and perhaps even violence," reads the concluding section of the paper.

This is a little troubling for a long-term resident of a southern state like this writer to read, particularly one like New Mexico where the statistics around violent crime may be troubling, but day-to-day interactions tend to be heavily influenced by a culture that largely avoids most types of confrontations, often to a fault.

Herein lies the danger of being painted with the very broad brush of social science. Fortunately, and to their credit, the scientists behind the CLASH model are enthusiastic about seeing the model tested in the real world.

"We believe CLASH can help account for differences in aggression and violence both within and between countries around the world," Van Lange said. "We think it provides a strong framework for understanding the violence differences we see around the world."

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