Van Lange et al. propose the CLASH model, in which a slow life strategy, characterized by future-oriented self-control, inhibits aggression in environments that have lower temperatures and larger seasonal variation in climate. Their model is based on Life History Theory (e.g., Figueredo et al. Reference Figueredo, Vásquez, Brumbach, Schneider, Sefcek, Tal, Hill, Wenner and Jacobs2006), which proposes that people adapt to environmental (un)harshness and (un)predictability by adopting either fast or slow life history strategies. CLASH focuses on humans, rather than other animals, perhaps because research on animals seems inconsistent with the CLASH model.

Contrary to Van Lange et al., research on individual differences in animals relates aggressiveness to predictable environments. It has been proposed that aggressive animals have the evolutionary advantage of being better able to compete with others for known resources in predictable and familiar environments. As such, aggressive animals easily develop routines, that is, a rather intrinsically driven rigid type of behavior, and show reduced impulse control in operant conditioning paradigms. Nonaggressive animals, on the other hand, react more strongly to environmental stimuli, that is, they show larger cue dependency. For that reason, Koolhaas et al. (Reference Koolhaas, Korte, De Boer, Van Der Vegt, Van Reenen, Hopster, De Jong, Ruis and Blokhuis1999) suggested the terms proactive coping, which they associated with higher levels of aggression, and reactive coping, associated with increased restraint and lower aggression.

Proactive and reactive coping styles represent fundamental biological characteristics that can be observed in many species. The optimal proportion of each trait in a population changes with the predictability and stability of the environment (Koolhaas et al. Reference Koolhaas, Korte, De Boer, Van Der Vegt, Van Reenen, Hopster, De Jong, Ruis and Blokhuis1999). The two traits evolved because reactive control of behavior is adaptive in unpredictable or changing environments, whereas proactive control is adaptive in predictable and stable environments (Tops et al. Reference Tops, Boksem, Quirin and Koole2014). Although proactive individuals lack flexibility, they tend to increase the predictability and stability of their environment through social dominance and by being in control. However, the exclusive association of aggression with proactive coping is nuanced by literature suggesting that proactive and reactive coping have been associated with differential types of aggression, namely proactive aggression which is ‘cold-blooded’ and supports goal-directed behavior, and reactive aggression which is associated with ‘hot-tempered’ emotional responses to threat (Kempes et al. Reference Kempes, Matthys, de Vries and van Engeland2005). However, we suggest that both types of aggression are lowered by future-oriented control in humans.

The association between proactive control and aggression described in the animal literature only applies to simpler adaptations to predictable environments that benefit from rigid routines (Del Giudice Reference Del Giudice, Gendolla, Tops and Koole2015; Tops Reference Tops2014). For more sophisticated, future-oriented control in predictable environments, lowered impulsivity and reduced overt aggressive behavior seems to be beneficial. The fact that most species do not engage in the kind of long-term planning typical of human adults may explain the apparent consistency of the animal literature, in which predictability is almost invariably associated with traits that indicate primacy of proactive control, including aggression. We proposed that evolution produced at least two more coping styles in humans (Tops Reference Tops2014). Because both those coping styles exploit the advantages of predictable environments and are associated with low aggression, their existence in humans reconciles CLASH with the animal literature on proactive coping styles.

First, we argued that human evolution favored the emergence of a more self-regulatory type of personality (Tops Reference Tops2014; Tops et al. Reference Tops, Montero Marín, Quirin, Kim, Reeve and Bong2016; Van der Linden et al. Reference Van der Linden, Dunkel, Beaver and Louwen2015), in which the aggressive control of the proactive trait is supplemented by a conscientious variant. This personality type exploits the advantages of collaboration and of moral and authority rule structures in order to protect obtained (in-group) resources from aggression and other threats. More specifically, predictability enables long-term investments and slow life history strategies (reduced impulsivity and aggression) if those investments can be protected against aggressive competition. Notably, the ‘cold winter theory’ claims that cold climates were a driving force in the evolution of predictive control by increasing selection of self-regulation, social constraints and future-orientation (Lynn Reference Lynn1987). However, similar to proactive personality, the conscientious, self-regulatory personality type is still associated with rigidity (Ferguson et al. Reference Ferguson, Semper, Yates, Fitzgerald, Skatova and James2014). For example, conscientiousness has been associated with higher probability of obsessive-compulsive disorder (Del Giudice Reference Del Giudice2014; Tops Reference Tops2014).

Second, we argued that in humans not only the conscientious coping style evolved, but also a coping style that makes use of more flexible predictive control. Although proactive animals seem to a certain degree able to exploit environmental predictability, humans appear to be able to go beyond such rigid predictive (i.e., proactive) control by using more flexible predictive control. Through increased encephalization, prolonged learning, and development of language, humans evolved flexible predictive control that could be applied in a wider variety of circumstances such as increased exchange with outgroups (Tops Reference Tops2014).

As we discussed, the distinction between proactive control on the one hand and, on the other hand, the conscientious, self-regulatory personality (Van der Linden et al. Reference Van der Linden, Dunkel and Petrides2016) and flexible predictive control, may explain why predictable environments are associated with impulsivity and aggression in animals, but with future-orientation in humans. By discriminating adaptations to predictable environments that are associated with either high or low aggression, we add some nuance to a simple predictable/slow strategy/low aggression – unpredictable/fast strategy/high aggression dichotomy. Resource availability, another key dimension of the environment that affects the development of life history strategies, shows similar complexities. Although low environmental resources tend to be associated with fast life strategies, energetic stress may cause the developing individual to shift toward a slower life history strategy. This translates into development of a more energy-sparing phenotype, including slower growth, delayed sexual maturation, and low fecundity (Ellis et al. Reference Ellis, Figueredo, Brumbach and Schlomer2009). The role of temperature in life history strategies is probably similarly complicated. Temperature is related to environmental resources and higher temperature is itself an environmental resource (Davis & Levitan Reference Davis and Levitan2005; IJzerman et al. Reference IJzerman, Coan, Wagemans, Missler, van Beest, Lindenberg and Tops2015a). Moreover, temperature has also been hypothesized in various ways to be implicated in increased encephalization (Naya et al. Reference Naya, Naya and Lessa2016), a development that seems crucial in flexible predictive control. The association with resources and encephalization suggests that higher temperature may be involved in mechanisms that increase aggression as well as in mechanisms that are aimed to control overt aggressive behavior.