There are a number of significant methodological and theoretical errors in the article by Van Lange and colleagues. We take one specific element as the focus of our response here: they fail to include historical, archaeological, and paleoanthropological data in their model. By not incorporating a “deep time” perspective, the hypothesis lacks the evolutionary baseline the authors seek to infer in validating the model. Data from human history directly challenge the validity of the basic assumption of the CLASH model that higher average temperature with smaller seasonal variation creates an evolutionary process biasing populations toward increased aggression.
If Van Lange et al. are correct about the relationships among aggression/violence, climate, and life history, we should find the majority of conflicts occurring in equatorial regions during prehistoric and historic times. This is not the case, as historical (e.g., Vikings, Mongols), recent (e.g., World War 1, World War 2), and archeological data attest (e.g., Ferguson Reference Ferguson and Fry2013). Such data illuminate a significant problem for the model. The authors fail to consider historical and cultural legacies and deep-time data relevant to the exhibition of aggression. Moreover, we wonder how the CLASH model, and its focus on countries as units of analysis, accounts for migration in recent and historical contexts, as genetic evidence points to substantial migration across latitudes throughout human history (Cavalli-Sforza et al. Reference Cavalli-Sforza, Menozzi and Piazza1996; Coop et al. Reference Coop, Pickrell, Novembre, Kudaravalli, Li, Absher, Myers, Cavalli-Sforza, Feldman and Pritchard2009; Templeton Reference Templeton1999). How many generations, under CLASH, would a group need to live in a lower latitude to evolve this new disposition? Likewise, would a person who grew up in the southern latitudes and then migrated to the north be more aggressive than a native-born northerner?
Indeed, we are surprised that for a paper making such grandiose claims, so few data are offered in support their assertions. They rely on citing compilation works or making assumptions such as “societies closer to the equator are also relatively harsh and unpredictable” (sect. 3.1, para. 7), without defining which societies these are or how parameters like unpredictability are measured. For a model that seeks to explain major behavioral components through causal links, we would hope to see more empirical support. For example, while they note that they only look at the Northern Hemisphere, they suggest that the model will hold for the Southern Hemisphere as well. But is this true? Figure 1 illustrates the homicide rate by country per year (between 2012 and 2014) per 100,000 inhabitants (data from the World Bank) by the latitude of the country (taken as the midpoint of the country). As can be seen, the data support the model in the northern hemisphere, with r
2 = 0.108. However, in the Southern Hemisphere the relationship does not hold (r
2 = 0.007). This suggests to us that other factors are in play.
Figure 1.
Homicide rate with respect to latitude for each country.
Variation in temperature may provide a perspective on environmental impact, but it does not take into account more accurate reflections of local ecologies such as rainy seasons, droughts, and other climatic events that could increase variability and its interplay with historical, political, and economic contexts. For example, the authors rely extensively on the meta-analyses of Burke et al. (Reference Burke, Hsiang and Miguel2015), whose data sets strongly indicate that economic impacts of climatic variation are critical in leading to conflicts and that such conflicts are tied to the relative importance of local agricultural production. Temperature out of context is meaningless.
For the CLASH model to reflect an actual outcome of natural selection processes, there must be a demonstrated connection between the ecological variables of interest (in their case temperature) and specific effects on fitness values of the populations of interest. Their limited attempt to connect with Life History Theory is not sufficient. By purposefully removing key historical and ecological variables and attempting to reduce the complex suites of behaviors in aggression and conflict to simplistic few-variant causal factors (average temperature and seasonal variation in temperature), the relevance of such gross model predictions is called into question.
There is substantial paleoanthropological research into the question of how climate affects humans and our ancestors. Adaptation to increasing climactic variability/unpredictability appears to be a core process for early Homo (Anton et al. Reference Anton, Potts and Aiello2014), suggesting that our species as a whole is pre-adapted to climate variability. Geographers and paleoclimatologists argue that economic transformations and conflicts are associated with temperature declines (Büntgen et al. Reference Büntgen, Kyncl, Ginzler, Jacks, Esper, Tegel, Heussner and Kyncl2013), whereas periods of prosperity are linked to wet and warm summers (Büntgen et al. Reference Büntgen, Tegel, Nicolussi, McCormick, Frank, Trouet, Kaplan, Herzig, Heussner, Wanner, Luterbacher and Esper2011). Moreover, the advent of agricultural communities at the beginning of the Holocene reduced the amount of resource unpredictability, but increased the opportunities and frequencies of conflict over resources (Ferguson Reference Ferguson and Fry2013, Bowles & Choi Reference Bowles and Choi2013), complicating the assertions of the proposed model.
To be fair, Van Lange et al. do offer a section on “caveats” recognizing the potential for socioeconomic and political-historical factors to play a role. But their treatment of these possibilities is superficial and relies on statements such as “over time, cultures have evolved such that economic growth and prosperity decrease as distance to the equator decreases” (sect. 5.1, para. 1). Archeologically and historically speaking this is false, as the Mayan, Incan, Egyptian, Majapahit, and many other major civilizations demonstrate. If the authors mean for the CLASH model to reflect only the past two to three centuries, then they cannot seriously propose that it reflects evolutionary processes. Many of the countries in equatorial regions today are postcolonial nations and, thus, have historical ties to other nations and economic processes that create and/or amplify a wide range of structural inequalities. These regions are often underdeveloped with respect to economic and health infrastructures, making it likely that historical contingencies (Furtado Reference Furtado1964), rather than climate, are particularly important causal factors in lower life expectancy, higher homicide rates, and more aggression. Until these confounding variables can be addressed, we remain skeptical of the CLASH model.
In sum, if a model seeks to be evolutionary, as the CLASH model does, data and theory that offer both temporal depth and evolutionarily relevant contexts cannot be ignored.
There are a number of significant methodological and theoretical errors in the article by Van Lange and colleagues. We take one specific element as the focus of our response here: they fail to include historical, archaeological, and paleoanthropological data in their model. By not incorporating a “deep time” perspective, the hypothesis lacks the evolutionary baseline the authors seek to infer in validating the model. Data from human history directly challenge the validity of the basic assumption of the CLASH model that higher average temperature with smaller seasonal variation creates an evolutionary process biasing populations toward increased aggression.
If Van Lange et al. are correct about the relationships among aggression/violence, climate, and life history, we should find the majority of conflicts occurring in equatorial regions during prehistoric and historic times. This is not the case, as historical (e.g., Vikings, Mongols), recent (e.g., World War 1, World War 2), and archeological data attest (e.g., Ferguson Reference Ferguson and Fry2013). Such data illuminate a significant problem for the model. The authors fail to consider historical and cultural legacies and deep-time data relevant to the exhibition of aggression. Moreover, we wonder how the CLASH model, and its focus on countries as units of analysis, accounts for migration in recent and historical contexts, as genetic evidence points to substantial migration across latitudes throughout human history (Cavalli-Sforza et al. Reference Cavalli-Sforza, Menozzi and Piazza1996; Coop et al. Reference Coop, Pickrell, Novembre, Kudaravalli, Li, Absher, Myers, Cavalli-Sforza, Feldman and Pritchard2009; Templeton Reference Templeton1999). How many generations, under CLASH, would a group need to live in a lower latitude to evolve this new disposition? Likewise, would a person who grew up in the southern latitudes and then migrated to the north be more aggressive than a native-born northerner?
Indeed, we are surprised that for a paper making such grandiose claims, so few data are offered in support their assertions. They rely on citing compilation works or making assumptions such as “societies closer to the equator are also relatively harsh and unpredictable” (sect. 3.1, para. 7), without defining which societies these are or how parameters like unpredictability are measured. For a model that seeks to explain major behavioral components through causal links, we would hope to see more empirical support. For example, while they note that they only look at the Northern Hemisphere, they suggest that the model will hold for the Southern Hemisphere as well. But is this true? Figure 1 illustrates the homicide rate by country per year (between 2012 and 2014) per 100,000 inhabitants (data from the World Bank) by the latitude of the country (taken as the midpoint of the country). As can be seen, the data support the model in the northern hemisphere, with r 2 = 0.108. However, in the Southern Hemisphere the relationship does not hold (r 2 = 0.007). This suggests to us that other factors are in play.
Homicide rate with respect to latitude for each country.
Variation in temperature may provide a perspective on environmental impact, but it does not take into account more accurate reflections of local ecologies such as rainy seasons, droughts, and other climatic events that could increase variability and its interplay with historical, political, and economic contexts. For example, the authors rely extensively on the meta-analyses of Burke et al. (Reference Burke, Hsiang and Miguel2015), whose data sets strongly indicate that economic impacts of climatic variation are critical in leading to conflicts and that such conflicts are tied to the relative importance of local agricultural production. Temperature out of context is meaningless.
For the CLASH model to reflect an actual outcome of natural selection processes, there must be a demonstrated connection between the ecological variables of interest (in their case temperature) and specific effects on fitness values of the populations of interest. Their limited attempt to connect with Life History Theory is not sufficient. By purposefully removing key historical and ecological variables and attempting to reduce the complex suites of behaviors in aggression and conflict to simplistic few-variant causal factors (average temperature and seasonal variation in temperature), the relevance of such gross model predictions is called into question.
There is substantial paleoanthropological research into the question of how climate affects humans and our ancestors. Adaptation to increasing climactic variability/unpredictability appears to be a core process for early Homo (Anton et al. Reference Anton, Potts and Aiello2014), suggesting that our species as a whole is pre-adapted to climate variability. Geographers and paleoclimatologists argue that economic transformations and conflicts are associated with temperature declines (Büntgen et al. Reference Büntgen, Kyncl, Ginzler, Jacks, Esper, Tegel, Heussner and Kyncl2013), whereas periods of prosperity are linked to wet and warm summers (Büntgen et al. Reference Büntgen, Tegel, Nicolussi, McCormick, Frank, Trouet, Kaplan, Herzig, Heussner, Wanner, Luterbacher and Esper2011). Moreover, the advent of agricultural communities at the beginning of the Holocene reduced the amount of resource unpredictability, but increased the opportunities and frequencies of conflict over resources (Ferguson Reference Ferguson and Fry2013, Bowles & Choi Reference Bowles and Choi2013), complicating the assertions of the proposed model.
To be fair, Van Lange et al. do offer a section on “caveats” recognizing the potential for socioeconomic and political-historical factors to play a role. But their treatment of these possibilities is superficial and relies on statements such as “over time, cultures have evolved such that economic growth and prosperity decrease as distance to the equator decreases” (sect. 5.1, para. 1). Archeologically and historically speaking this is false, as the Mayan, Incan, Egyptian, Majapahit, and many other major civilizations demonstrate. If the authors mean for the CLASH model to reflect only the past two to three centuries, then they cannot seriously propose that it reflects evolutionary processes. Many of the countries in equatorial regions today are postcolonial nations and, thus, have historical ties to other nations and economic processes that create and/or amplify a wide range of structural inequalities. These regions are often underdeveloped with respect to economic and health infrastructures, making it likely that historical contingencies (Furtado Reference Furtado1964), rather than climate, are particularly important causal factors in lower life expectancy, higher homicide rates, and more aggression. Until these confounding variables can be addressed, we remain skeptical of the CLASH model.
In sum, if a model seeks to be evolutionary, as the CLASH model does, data and theory that offer both temporal depth and evolutionarily relevant contexts cannot be ignored.