It is encouraging to see life history thinking being incorporated into a broad, societal-level model explaining how differences in average and variation in local temperature are related to specific psychological and social processes, which in turn affect reactive aggression and violence. The aggression/violence area has needed, and will benefit from, an infusion of more evolutionary thinking.

In this commentary we address two questions: (1) From a life history perspective, is the drive in many young men to gain status and amass resources, which often may involve direct competition with outgroups, one of the key factors underlying the CLASH model? (2) Why is there so much variation in reactive aggression/violence between people living in the same environment if local temperature and seasonal variation are such fundamental predictors of aggression/violence?

Question 1. In the target article, the authors cite an important finding that receives insufficient theoretical attention. In a recent meta-analysis, Burke et al. (Reference Burke, Hsiang and Miguel2015) reported that a one standard deviation increase in local temperature predicts an 11.3% increase in intergroup conflict, but only a 2.1% increase in interpersonal conflict. Moreover, these effects are confined primarily to visible, “reactive” forms of aggression/violence rather than other types of offenses or conflicts.

This significant difference provides a telltale sign that the deeper evolutionary reason for some of the effects anticipated by the CLASH model may center on how certain people – especially young, aspiring males with few resources and limited options – may try to increase their reproductive fitness at the expense of members of outgroups with whom they are competing in high average temperature/low variability temperature environments. In these environments, social status is important and resources are scarce and difficult to obtain and maintain, so finding a way to achieve higher social status and secure resources is paramount. One way to elevate one's status and rank in the ingroup, especially if one is a young male of mating age who lacks status and resources, is to engage in risky, opportunistic actions that would improve one's position within the ingroup relative to competing outgroups. Such actions may, at times, involve intimidation, aggression, or violence directed at outgroup members during scrambles for status or limited resources (Wilson & Daly Reference Wilson and Daly1997). Young males who engage in such behaviors that yield successful outcomes should become leaders of their ingroups, which typically would translate into greater reproductive fitness during our ancestral past (Puts et al. Reference Puts, Bailey, Reno and Buss2016). This should be particularly true in pathogen-ridden environments, in which leaders engaged in very difficult or highly taxing behaviors perceived by others – especially by potential mates – as being “honest signals” (Zahavi Reference Zahavi1975) of their health, stamina, and durability (Gangestad & Simpson Reference Gangestad and Simpson2000). Because women in evolutionary history did not need to gain status or accrue resources before reproducing, this pattern should be specific to males.

This account provides a good explanation for the aggressive/violent actions of young, resourceless males, but not for females. The vast majority of reactive aggression/violence, however, is perpetrated by young men who lack status and resources and have limited options to achieve them (Daly & Wilson Reference Daly and Wilson1988). Deeper and clearer evolutionary thinking could be infused into the CLASH model if Burke et al.'s intergroup conflict versus interpersonal conflict findings are integrated with sexual selection principles that address gender differences (see Trivers Reference Trivers and Campbell1972).

This focus on the “fast” side of the temperature→reactive aggression/violence equation does not diminish the theoretical importance of the “slow” side. From a life history perspective, large seasonal swings in temperature should require better planning for the future, greater impulse control, and more sustained efforts to be cooperative, not only with ingroup members, but also with outgroups with which trust and reciprocal exchanges can be developed. All of these tendencies should have increased reproductive fitness in both sexes in these arduous environments. Thus, different evolutionary forces may have shaped the temperature→reactive aggression/violence link: one driven primarily by young males seeking to gain status and resources in pathogen-prevalent, intergroup-competitive, and high temperature/low temperature variability environments, and another driven by males and females who had to survive and successfully reproduce in equally challenging “heat and eat” environments.

Question 2. At the societal level, reactive aggression/violence is clearly more prevalent in hotter environments with smaller seasonal changes in temperature. Nevertheless, there is a great deal of variation in reactive aggression/violence between people who live in the same environment. Some people display reactive aggression/violence relatively frequently, whereas others never do. The CLASH model does not explain these individual-level effects, but other life history models do.

As discussed in the target article, different programs of research inspired by Ellis et al.'s (Reference Ellis, Figueredo, Brumbach and Schlomer2009) distinction between harsh and unpredictable environments have revealed that individuals exposed to more harshness (e.g., lower-socioeconomic-status environments) and/or greater unpredictability (e.g., more chaotic home environments) early in life are more likely to engage in short-term planning, take more and larger risks, prefer immediate gratification over delayed rewards, and be more aggressive (e.g., Frankenhuis et al. Reference Frankenhuis, Panchanathan and Nettle2016; Griskevicius et al. Reference Griskevicius, Tybur, Delton and Robertson2011; Reference Griskevicius, Ackerman, Cantu, Delton, Robertson, Simpson, Thompson and Tybur2013; Nettle Reference Nettle2010; Simpson et al. Reference Simpson, Griskevicius, Kuo, Sung and Colling2012). Much of this research is consistent with Belsky et al.'s (Reference Belsky, Steinberg and Draper1991) Evolutionary Model of Social Development. According to this model, the primary evolutionary function of early social experience is to prepare children for the social and physical environments they are likely to inhabit during their lifetime. Males and females exposed to high levels of harshness and/or unpredictability should receive less sensitive/more rejecting parenting, which generates insecure attachment working models. Insecure models, in turn, should speed up physical maturation, resulting in earlier sexual activity, short-term/unstable romantic pair bonds, and less parental investment (a fast orientation). Those exposed to lower levels of stress early in life should enact a slow orientation, which culminates in later sex, long-term/stable pair bonds, and greater parental investment. Mounting evidence supports this model (Simpson & Belsky Reference Simpson, Belsky, Cassidy and Shaver2016; Szepsenwol et al. Reference Szepsenwol, Simpson, Griskevicius and Raby2015).

To obtain a more complete and nuanced understanding of why temperature is related to reactive aggression/violence, societal-level models such as CLASH should be melded with individual-level life history models.