Van Lange et al. suggest that environmental harshness and unpredictability, associated with low-latitude equatorial environments, lead to faster life history strategies, of which increased violence and aggression is one consequence. In general, we agree that factors that correlate with latitudinal climatic variation might influence relative frequencies of fast and slow life history strategies and that this could plausibly lead to different rates of aggression and violence in different regions across the world. The CLASH model, however, depends on questionable characterizations of what constitutes harsh, unpredictable environments and shaky assumptions about Life History Theory, and is inconsistent with patterns of violent crime rates in many regions of the world.
The authors make a very specific argument about environmental predictability and harshness: they equate high temperatures and low seasonal variation in temperature with high levels of harshness and unpredictability. Conversely, high seasonal variation, including the harsh winters that occur as one approaches the poles, are described as predictable, hence less harsh. Although environmental challenges apart from temperature and its annual fluctuations will undoubtedly contribute to harshness, Van Lange et al. argue that temperature is the environmental variable with the most explanatory power regarding violence and aggression, so for the moment we will limit our criticisms to that. It seems somewhat counterintuitive to us to assert that low seasonal variations in temperature automatically produce an unpredictable environment, which would render planning for the future an unprofitable activity. Harsh northern winters, on the other hand, may well provide selection pressure for future planning – not because the environment is mild and so a long, future-focused, life history strategy generally is adaptive – but simply because, if you don't plan and stockpile, you die. It seems more valid to argue that the harshness of extreme temperature fluctuations necessitates the capacity to plan, rather than to argue that the supposed harshness and unpredictability of annually consistent temperatures preclude planning. Indeed, strong seasonality has been recently associated with rises in violent crime because groups use warm periods to loot in preparation for the coming harsh periods (Landis Reference Landis2014).
As the authors argue, one factor that does contribute to environmental harshness in the tropics is pathogen prevalence; this is the main “harsh” environmental factor associated with equatorial regions. The relationship between pathogens and latitude, however, is not straightforward. Disease is more prevalent at low latitude only where it is wet. Deserts and other arid environments have relatively low disease prevalence (Stromberg Reference Stromberg1997), and in many other respects, equatorial regions (particularly wet ones) are less harsh, with plentiful, year-round resources.
In addition to theoretical difficulties, the CLASH model is enunciated with reference to only a subset of the available empirical evidence. The evidence reviewed is derived only from the Northern Hemisphere, because most people live in this hemisphere and it has therefore been the focus of most research. Van Lange et al. omit some awkward data points even from the Northern Hemisphere (Alaska has one of the highest rates of violent crime in the United States, and Russia is considered an outlier), and the Southern Hemisphere provides very obvious exceptions to the latitudinal pattern of violence: Australia and New Zealand have low levels of violence, whereas South Africa (which is at a similar latitude) has much higher levels (with African countries between South Africa and the equatorial countries much lower). Melanesian and Polynesian islands have relatively high levels of some kinds of violence, whereas more equatorial Southeast Asia has low levels of violence.
In an analogous kind of global analysis, patterns of women's preferences for masculinity in men's faces were attributed to patterns of disease prevalence (DeBruine et al. Reference DeBruine, Jones, Crawford, Welling and Little2010), with women in areas of higher disease prevalence adopting a faster life history strategy and a preference for cues to good genes. A subsequent reanalysis of the data, however, showed that the pattern was more parsimoniously explained by the level of economic inequality present in each country (measured by the Gini coefficient), leading to increased levels of competition in equatorial countries (Brooks et al. Reference Brooks, Scott, Maklakov, Kasumovic, Clark and Penton-Voak2010). Given the pattern of high-latitude countries in the Northern Hemisphere being characterised by relatively high wealth and low wealth inequality, it may be that the fast life history strategies associated with equatorial, high-violence countries, may also be attributed to this increased competition over resources.
Although there is a broad global pattern of lower levels of violence in places of higher latitude (with some important exceptions), this is not obviously a consequence of such climates proximally selecting for slow life history strategies, as the model requires. One alternative ultimate-level explanation is that surviving harsh, resource-poor winters requires high levels of cooperation, and this is what has led to lower levels of interpersonal violence in such regions, as interpersonal violence interferes with cooperative enterprises.
Such adaptation might result from cultural practices, selection on dispositional characteristics, or both. And high-latitude societies have exported both their practices and institutions. Some former European colonies have low violence, possibly because democratic governance took hold in those places, irrespective of their latitude. Former colonies where economic and political institutions are extractive rather than inclusive are among the least prosperous, successful, and peaceful places today (Acemoglu & Robinson Reference Acemoglu and Robinson2012).
In conclusion, although we agree that the increased violence in equatorial countries may be partially attributable to fast life history strategies, we question whether the “harsh” environment proposed as the cause of this life history variation is plausibly disease prevalence. We also suggest that other factors, not included in the model, might better account for the generally lower levels of violence at high latitudes, as well as for the complex mosaic of violence patterns across the globe.
Van Lange et al. suggest that environmental harshness and unpredictability, associated with low-latitude equatorial environments, lead to faster life history strategies, of which increased violence and aggression is one consequence. In general, we agree that factors that correlate with latitudinal climatic variation might influence relative frequencies of fast and slow life history strategies and that this could plausibly lead to different rates of aggression and violence in different regions across the world. The CLASH model, however, depends on questionable characterizations of what constitutes harsh, unpredictable environments and shaky assumptions about Life History Theory, and is inconsistent with patterns of violent crime rates in many regions of the world.
The authors make a very specific argument about environmental predictability and harshness: they equate high temperatures and low seasonal variation in temperature with high levels of harshness and unpredictability. Conversely, high seasonal variation, including the harsh winters that occur as one approaches the poles, are described as predictable, hence less harsh. Although environmental challenges apart from temperature and its annual fluctuations will undoubtedly contribute to harshness, Van Lange et al. argue that temperature is the environmental variable with the most explanatory power regarding violence and aggression, so for the moment we will limit our criticisms to that. It seems somewhat counterintuitive to us to assert that low seasonal variations in temperature automatically produce an unpredictable environment, which would render planning for the future an unprofitable activity. Harsh northern winters, on the other hand, may well provide selection pressure for future planning – not because the environment is mild and so a long, future-focused, life history strategy generally is adaptive – but simply because, if you don't plan and stockpile, you die. It seems more valid to argue that the harshness of extreme temperature fluctuations necessitates the capacity to plan, rather than to argue that the supposed harshness and unpredictability of annually consistent temperatures preclude planning. Indeed, strong seasonality has been recently associated with rises in violent crime because groups use warm periods to loot in preparation for the coming harsh periods (Landis Reference Landis2014).
As the authors argue, one factor that does contribute to environmental harshness in the tropics is pathogen prevalence; this is the main “harsh” environmental factor associated with equatorial regions. The relationship between pathogens and latitude, however, is not straightforward. Disease is more prevalent at low latitude only where it is wet. Deserts and other arid environments have relatively low disease prevalence (Stromberg Reference Stromberg1997), and in many other respects, equatorial regions (particularly wet ones) are less harsh, with plentiful, year-round resources.
In addition to theoretical difficulties, the CLASH model is enunciated with reference to only a subset of the available empirical evidence. The evidence reviewed is derived only from the Northern Hemisphere, because most people live in this hemisphere and it has therefore been the focus of most research. Van Lange et al. omit some awkward data points even from the Northern Hemisphere (Alaska has one of the highest rates of violent crime in the United States, and Russia is considered an outlier), and the Southern Hemisphere provides very obvious exceptions to the latitudinal pattern of violence: Australia and New Zealand have low levels of violence, whereas South Africa (which is at a similar latitude) has much higher levels (with African countries between South Africa and the equatorial countries much lower). Melanesian and Polynesian islands have relatively high levels of some kinds of violence, whereas more equatorial Southeast Asia has low levels of violence.
In an analogous kind of global analysis, patterns of women's preferences for masculinity in men's faces were attributed to patterns of disease prevalence (DeBruine et al. Reference DeBruine, Jones, Crawford, Welling and Little2010), with women in areas of higher disease prevalence adopting a faster life history strategy and a preference for cues to good genes. A subsequent reanalysis of the data, however, showed that the pattern was more parsimoniously explained by the level of economic inequality present in each country (measured by the Gini coefficient), leading to increased levels of competition in equatorial countries (Brooks et al. Reference Brooks, Scott, Maklakov, Kasumovic, Clark and Penton-Voak2010). Given the pattern of high-latitude countries in the Northern Hemisphere being characterised by relatively high wealth and low wealth inequality, it may be that the fast life history strategies associated with equatorial, high-violence countries, may also be attributed to this increased competition over resources.
Although there is a broad global pattern of lower levels of violence in places of higher latitude (with some important exceptions), this is not obviously a consequence of such climates proximally selecting for slow life history strategies, as the model requires. One alternative ultimate-level explanation is that surviving harsh, resource-poor winters requires high levels of cooperation, and this is what has led to lower levels of interpersonal violence in such regions, as interpersonal violence interferes with cooperative enterprises.
Such adaptation might result from cultural practices, selection on dispositional characteristics, or both. And high-latitude societies have exported both their practices and institutions. Some former European colonies have low violence, possibly because democratic governance took hold in those places, irrespective of their latitude. Former colonies where economic and political institutions are extractive rather than inclusive are among the least prosperous, successful, and peaceful places today (Acemoglu & Robinson Reference Acemoglu and Robinson2012).
In conclusion, although we agree that the increased violence in equatorial countries may be partially attributable to fast life history strategies, we question whether the “harsh” environment proposed as the cause of this life history variation is plausibly disease prevalence. We also suggest that other factors, not included in the model, might better account for the generally lower levels of violence at high latitudes, as well as for the complex mosaic of violence patterns across the globe.