In the aftermath of violent acts that result in the loss of human life, one of the most pressing questions for survivors and witnesses is simply: why?
Figuring out what would lead someone to take another's life is rarely easy and the answers are usually highly specific to the context in which in the violence occurred. But for researchers tasked with analyzing patterns of violence on a macroscale, an intriguing trend has emerged in the last two decades: instances of violence and aggression are generally higher in countries that are close to the equator.
There are exceptions to this observation of course and the levels of violence can vary wildly within a country, but as was shown in a 2015 meta-analysis on conflict and climate, temperature was associated with violence in 20 out of the 24 studies surveyed for the analysis. Two main theories, known as the General Aggression Model and Routine Activity Theory, have been used as explanations for this strong correlation between high temperatures and violence.
The General Aggression Model boils down to the observation that being hot is uncomfortable and irritating, and irritated people are more likely to act violently or aggressively towards others. On the other hand, Routine Activity Theory posits that those who live in warm climates tend to spend more time outside, which puts them into more contact with other people and thus increases the number of opportunities for conflict.
While both of these theories seem pretty intuitive, they still can't explain extreme acts of violence such as murder, or why there is more violence when it's 90 F than when it's 70 F, even though people are likely to be outdoors in both temperatures. The lack of satisfactory answers to these questions recently led a team of psychologists at Vrije Universiteit Amsterdam to propose a more nuanced theory of why high temperatures get people heated.
Known as Climate Aggression and Self-control in Humans (CLASH), the new theory suggests that it's not just high temperatures that lead to more violence, but also the fact that there is less seasonal variation in these temperatures in equatorial regions.
The thinking behind CLASH is this: consistently warm temperatures in these regions means there is less need to plan ahead for significant temperature changes between winter and summer. This means that rather than thinking about the future, the people in these regions are more focused on the present which results in less self-control. With less concern about the future coupled with heat-induced irritability, people will tend to react to situations more quickly in a violent or aggressive manner.
"Strong seasonal variation in temperature affects culture in powerful ways," said Paul van Lange, a professor of psychology at Vrije Universiteit Amsterdam. "But it does shape how much a culture values time and self-control. 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."
Van Lange and his colleagues are quick to point out that their theory is not deterministic—consistently warm temperatures don't force people to act violently. Rather, their theory points to the large role that climate has in shaping human culture, and the way a person reacts to a given situation is largely a product of the culture they were raised in.
Climate change-fueled violence is on the rise around the world, and although CLASH is a new theory that awaits empirical validation, the development of new theoretical frameworks is an important part of figuring out how to respond to and prevent climate-related violence.
"Climate doesn't make a person, but it is one part of what influences each of us," said van Lange. "We believe CLASH can help account for differences in aggression and violence both within and between countries around the world."