In their target article, Van Lange et al. argue that cooler temperatures and greater seasonal variation encourage future planning and self-control in countries further from the equator, leading to reduced reactive violence. Van Lange et al. draw together research from evolutionary, social, and cognitive psychology in pursuit of an integrative model. However, we do not find their model compelling. Firstly, we point out an alternative role for explicit mental planning. Secondly, we note that the authors are not clear what kind of explanation they advance and, thus, what predictions the model makes. Finally, we note some apparent inconsistencies and empirical holes in their argument.

Although people vary in their tendency to make future-oriented decisions (Kirby et al. Reference Kirby, Petry and Bickel1999; Zimbardo et al. Reference Zimbardo, Keough and Boyd1997), the capacity to travel mentally in time is a universal human ability among healthy adults, with undeniable adaptive advantages (Suddendorf & Corballis Reference Suddendorf and Corballis2007). The ability to imagine future situations and organise current actions accordingly has been called episodic foresight (Suddendorf & Moore Reference Suddendorf and Moore2011). People frequently rely on episodic foresight when making various kinds of decisions (e.g., Gilbert & Wilson Reference Gilbert and Wilson2007), including, as Van Lange et al. acknowledge, social ones (e.g., Boyer Reference Boyer2008). Van Lange et al. note that “The anticipation of future interaction is a powerful determinant of unselfish and cooperative behavior in social dilemmas” (sect. 3.2, para. 3). Indeed, as economists and psychologists have realised, the anticipated costs of antisocial behaviours can reduce a propensity for so-called “short-sighted” or “impulsive” social transgressions, like stealing and aggression that may accrue immediate benefits, for instance, in terms of material wealth or status (Boyer Reference Boyer2008; Frank Reference Frank1988).

Thus, one of the reasons humans avoid reactive violence is that the delayed interpersonal costs of doing so can be foreseen. However, we think the role of mental time travel into the future does not end here. It may have the opposite effect when people imagine futures that are volatile, uncertain, or harsh. In those circumstances, delayed relational and coalitional costs of immediate violence may be downplayed because they are foreseen as less likely to materialise, or less dramatic against the harsh backdrop of one's expected future (see Bolland Reference Bolland2003; Brezina et al. Reference Brezina, Tekin and Topalli2009). For this reason, a seeming “failure” of self-control in reactive violence may sometimes be caused not by a disinclination to plan ahead, as Van Lange et al. imply, but by very virtue of this ability. In other words, prudent foresight, in certain circumstances, should lead to a general prioritisation of the present (Bulley et al. Reference Bulley, Henry and Suddendorf2016; see also Daly & Wilson Reference Daly and Wilson2005).

A second concern is that it is not clear what reasons the authors propose for the apparent links between average temperature/seasonal variation and life history, time perspective, self-control, and aggression. Are the purported relationships driven by explicit mental reasoning (as discussed previously), individual learning, cultural evolution, an evolved genetic predisposition or calibration mechanism, or some combination of these factors? At times Van Lange et al. point to individual reasoning and foresight, for instance, when they write that individuals “realize that they need to plan and prepare for the next season” (sect. 3.1, para. 8). At other times, they appeal to evolutionary adaptation or developmental plasticity: “Average temperature and seasonal variation in temperature have shaped the evolution and development of differential adaptation in terms of life strategy, time orientation, and self-control” (sect. 3.2, para. 1). And yet elsewhere, they seem to appeal to cultural evolution: “lower temperatures and especially greater seasonal variation in temperature helps individuals and societies evolve to be less aggressive” (sect. 3, para. 4; all emphases added).

Aside from inappropriately framing evolutionary processes in terms of goal directedness (e.g., that the environment “helps” people to evolve in a certain way), such statements fail to carefully delineate proximate (mechanistic/developmental) and ultimate (phylogenetic/functional) explanations, a practice that is critical when making evolutionary arguments (Mayr Reference Mayr1961; Tinbergen Reference Tinbergen1963). Given the potentially socially divisive nature of some possible interpretations of Van Lange et al.'s propositions, it seems particularly important to be clear about what kind of explanation the authors advance and, hence, what testable predictions follow (e.g., if their proposed explanation is at the phylogenetic level, it could be tested with genetic or twin studies).

The authors also apply their argument inconsistently. They do not adequately explain why seasonal temperature variation should encourage planning more than other important predictable environmental stressors. For example, many hot countries north of the equator are subject to seasonably variable, but reasonably predictable, precipitation (Brown & Lall Reference Brown and Lall2006), leading to significant water stress (Oki & Kanae Reference Oki and Kanae2006), a fact that Van Lange et al. acknowledge. Nonetheless, they choose not to focus on the effects of variability in rainfall, reasoning that (1) the effects of temperature have been more thoroughly examined in the literature, (2) the associations between temperature and conflict appear to be stronger than those between rainfall and conflict, and (3) for most countries, temperature varies more predictably than rainfall. Whilst these might be good reasons to focus on the effects of temperature variability, they are not good reasons to ignore other climatic variables, which, by applying the authors' logic, should be important. In avoiding discussion of the effects of rainfall, as well as other variables, Van Lange et al. fail to answer a key question: Why shouldn't the predictable stressors of hot climates also engender planning and self-control as per the predictable stressors of cooler climates?

There is a risk that ambiguous reasoning, aired in an esteemed journal such as this, will play into the hands of old racist prejudices about evolved inferiority of certain peoples. So we want to end this commentary with a warning that, in addition to the theoretical shortcomings already discussed, there are large empirical holes in the authors' case. For example, the target article avoids adequate consideration of the historical contexts of the regions in question (e.g., the profound consequences of slavery and colonisation for equatorial countries [see Diamond Reference Diamond1999]). They also sidestep potential counterexamples from near the equator (e.g., the peaceful nature of places such as Singapore [The World Bank 2013]), from history (e.g., the simultaneous planning successes and extreme violence of ancient Mesoamerican societies [see Harner Reference Harner1977]), and from prehistory (e.g., that the extended time perspective characteristic of our species arguably began to evolve in African savannah–dwelling hominins [see McBrearty & Brooks Reference McBrearty and Brooks2000; Suddendorf & Corballis Reference Suddendorf and Corballis2007]).