Researchers say the combination of high temperatures and lack of seasonal variation causes people to lead 'faster' lifestyles, contributing to more aggression and violence, and say it could get worse as global warming causes temperature to rocket.

Researchers from Vrije Universiteit Amsterdam developed the 'CLASH' model – CLimate, Aggression, and Self-control in Humans – to understand why violent crime is so high in hot climates.The model suggests that hot climates with little variation in temperature across the seasons cause people to put less focus on the future and behave with less self-control. This, in turn, leads to higher aggression and violence. 

Previous studies have linked violence and aggression simply to hot climates, but the two leading explanations of why that is so aren't satisfactory said Brad Bushman, a professor of psychology at The Ohio State University and VU Amsterdam.

The General Aggression Model, which Bushman helped to develop, attributes the aggression in hot climates to discomfort and irritation. But that doesn't explain more extreme acts, such as murder. A second theory, the Routine Activity Theory, says that warm weather leads people to be outdoors more often, thus creating more opportunities for conflict. But, this doesn't explain why violence increases as the temperatures grow hotter, with 95 degrees seeing more violence than 75.In the new model, the researchers consider lack of seasonal variation a factor as well.

'Less variation in temperature, combined with heat, brings some measure of consistency to daily life,' said Maria I. Rinderu of VU.

As a result of this, the researchers say people have less of a need to plan ahead for weather differences, causing them to be less concerned about the future, and have less need for self-control.

'Strong seasonal variation in temperature affects culture in powerful ways,' said Van Lange.

'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.'

'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.' Moving forward, the researchers will conduct studies to prove their model – but, they say substantial evidence already goes to support it. '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.'