Researchers have long struggled to explain why some violent crime rates are higher near the equator than other parts of the world. Now, a team of researchers have developed a model that could help explain why.

The researchers believe that hot climates and less variation in seasonal temperatures leads to a faster life strategy, less focus on the future, and less self-control -- all of which contribute to more aggression and violence.

"Climate shapes how people live, it affects the culture in ways that we don't think about in our daily lives," said Brad Bushman, co-author of the study and professor of communication and psychology at The Ohio State University.

The researchers, which included Maria I. Rinderu of VU, call the new model CLASH (CLimate Aggression, and Self-control in Humans). They describe the CLASH model in an online article in the journal Behavioral and Brain Sciences.

Many studies have shown that levels of violence and aggression are higher in hot climates, according to the researchers.

"But the two leading explanations of why that is so aren't satisfactory," Bushman said.

The General Aggression Model (which Bushman helped develop) suggests hot temperatures make people uncomfortable and irritated, which makes them more aggressive. "But that doesn't explain more extreme acts, such as murder," he said.

That means there is less need to plan for large swings between warm and cold weather. The result is a faster life strategy that isn't as concerned about the future and leads to less need for self-control.

"Strong seasonal variation in temperature affects culture in powerful ways. 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.

People living in these climates are oriented to the present rather than the future and have a fast life strategy -- they do things now.

With a faster life strategy and an orientation toward the present, people have to practice less self-control, he said. That can lead people to react more quickly with aggression and sometimes violence.

The theory is not deterministic and isn't meant to suggest that people in hotter, consistent climates can't help themselves when it comes to violence and aggression.

"How people approach life is a part of culture and culture is strongly affected by climate," Van Lange said. "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," he said.

Since CLASH is a new theory, studies have to be done to prove it is correct. But Bushman said a lot of evidence already suggests that the theory may be on to something.

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