Summary: New research suggests that hotter temperatures and climates with less seasonal variation are associated with greater aggression and violence.
Source: Ohio State University
Climate, Behavior, and the CLASH Model
A team of psychologists has proposed a new framework to explain why violent crime and aggressive behavior tend to be more common in regions nearer the equator. Moving beyond the simple observation that heat increases irritability, the new model—called CLASH (CLimate, Aggression, and Self-control in Humans)—links climate patterns to life strategies, time orientation, and self-control, which in turn influence levels of aggression.
Lead author Paul van Lange of the Vrije Universiteit Amsterdam and co-authors Maria I. Rinderu and Brad J. Bushman outline this model in Behavioral and Brain Sciences. Their work synthesizes cross-cultural and cross-regional evidence connecting temperature, seasonal variation, and social behavior.
Why temperature alone is not the full explanation
Previous explanations for higher aggression in warmer regions have focused on two main ideas. The General Aggression Model suggests that high temperatures create discomfort and irritation, increasing the risk of aggressive responses. Routine Activity Theory argues that warmer weather encourages people to spend more time outdoors and interact socially, which increases opportunities for conflict.
Both explanations capture part of the picture, but they leave important questions unanswered. For example, they do not fully account for why violence rises more sharply at very high temperatures than at moderately warm conditions, nor why regions with consistently warm climates but little seasonal change often show higher baseline levels of aggression.
CLASH: climate shapes life strategy, time perspective, and self-control
The CLASH model proposes that it’s not only mean temperature that matters, but also the degree of seasonal variation. In climates with high year-round heat and little seasonal fluctuation, daily life tends to be more stable and predictable in terms of temperature. That stability reduces the need for long-term planning around cold seasons—such as stockpiling food, preparing winter clothing, or managing agricultural cycles—and this shapes cultural practices and individual life strategies.
People in climates with less seasonal variation often develop a faster life strategy: greater emphasis on immediate needs and rewards, earlier and more frequent childbearing, and weaker emphasis on punctuality or long-term planning. This present-focused orientation reduces the everyday demand for self-control, which can make impulsive and aggressive responses more likely when conflicts arise.
By contrast, regions with substantial seasonal swings—colder winters and warmer summers—require planning, preparation, and delayed gratification. Those environmental pressures support slower life strategies, a stronger future orientation, and greater habitual self-control, which tend to inhibit aggressive behavior.
Culture, behavior, and non-determinism
The CLASH model is not deterministic. Climate is presented as one influential factor among many that shape culture and individual behavior. Van Lange emphasizes that climate “doesn’t make a person,” but it contributes to cultural norms and social practices that influence how people think about time, resources, and risk.
For example, in hotter climates people may also face immediate environmental risks such as disease vectors or venomous animals, which can heighten present-focused concerns. These pressures reinforce behaviors that prioritize the here-and-now, potentially increasing the likelihood of impulsive aggression.
Evidence and implications
The authors argue that CLASH integrates and extends existing theories by explaining both cross-regional and within-country variation in aggression and violence. They note numerous empirical findings consistent with their proposals—higher rates of aggression closer to the equator, relationships between seasonal variation and cultural practices, and links between life-history strategies and self-control—while calling for further research to test CLASH directly.
Bushman, a co-author and professor at Ohio State University, suggests the model provides a useful framework for understanding global differences in violence and for guiding future studies that examine how climate interacts with social, economic, and institutional factors to influence aggression.
About the research
The CLASH model—CLimate, Aggression, and Self-control in Humans—was proposed by Paul A. M. van Lange, Maria I. Rinderu, and Brad J. Bushman and published online in Behavioral and Brain Sciences in 2016. The model highlights how lower average temperatures and greater seasonal variation encourage slower life strategies, future orientation, and stronger self-control, all factors that help inhibit aggression and violence.
While the theory synthesizes existing findings and offers a coherent explanation for geographic differences in aggression, the authors call for empirical tests that can directly assess the model’s predictions across cultures and climates.
Abstract
Differences in aggression and violence within and between countries are substantial, and a consistent pattern shows higher levels of aggression nearer the equator. Existing theories emphasize immediate temperature effects or increased social interaction in warm weather, but they do not fully explain broader geographical patterns. The CLASH model proposes that climate—particularly lower temperatures and greater seasonal variation—promotes slower life-history strategies, future orientation, and greater self-control, which reduce aggression. Conversely, hot climates with little seasonal change foster faster life strategies, present orientation, and weaker self-control, which increase the likelihood of aggression and violence. CLASH offers a theoretical framework for understanding how climate shapes individual and societal differences in aggressive behavior.
Citation: Paul A. M. van Lange, Maria I. Rinderu, Brad J. Bushman. “Aggression and Violence Around the World: A Model of CLimate, Aggression, and Self-control in Humans (CLASH).” Behavioral and Brain Sciences. Published online May 23, 2016. DOI: 10.1017/S0140525X16000406