Life history theory is a theoretical framework for addressing how and why organisms, including humans, allocate resources to conflicting life tasks (Del Giudice et al. Reference Del Giudice, Gangestad, Kaplan and Buss2015; Roff Reference Roff2002; Sterns Reference Sterns1992). Life history theory is complex and rich in its predictions but is often oversimplified when applied to human psychology and behavior. Van Lange et al. (2016) also present an oversimplification of life history theory as the foundation for the CLASH model proposed to explain aggression and violence across the world. In particular, the model incorrectly specifies how environmental harshness and unpredictability affect life history strategies and behavioral outcomes. Here, we discuss three misunderstandings of life history theory expressed in the target article.
First, the CLASH model appears to conflate the constructs of environmental harshness and environmental unpredictability into a single predictor of individual variation in life history outcomes (e.g., aggression). The model correctly identifies that both environmental harshness and environmental unpredictability function to regulate life history strategies. The model incorrectly implies, however, that harshness and unpredictability are dependent constructs. Ellis et al.'s. (Reference Ellis, Figueredo, Brumbach and Schlomer2009) dimensions of environmental risk – harshness and unpredictability – are independent constructs that exert unique influences on individual variation in life history strategies. The CLASH model of aggression specifies that environmental harshness and environmental unpredictability determine the overall stress of the environment. In turn, this overall environmental stress is predictive of aggression. Environmental harshness and environmental unpredictability can be positively associated, such that a given environment can be harsh and unpredictable. The CLASH model, however, does not account for, or acknowledge, that (1) dimensions of environmental risk can be inversely associated (e.g., a predictable but harsh environment), and (2) environments characterized as harsh or unpredictable are not hypothesized to be exclusively associated with fast life histories. Ellis et al. (Reference Ellis, Figueredo, Brumbach and Schlomer2009) specify the conditions under which high environmental harshness and greater environmental unpredictability facilitate the adoption of either fast or slow life history strategies (pp. 218, 230). Whether environmental harshness or environmental unpredictability is associated with faster life history strategies is determined by age-specific rates of morbidity and mortality. Depending on whether external threats resulting from ecological factors exert greater influence on (or variation of) juvenile or adult morbidity and mortality rates, environmental harshness and unpredictability can facilitate faster or slower life histories. The CLASH model does not specify whether ecological conditions of temperature affect threats to juvenile or adult morbidity and mortality, or both. And neither does the CLASH model specify whether or how average temperature or variation in temperature differentially affect juvenile and adult morbidity and mortality.
Second, the CLASH model assumes that environmental harshness and environmental unpredictability exert similar and equal influence on specific life history outcomes over the life span. However, research has demonstrated that environmental harshness and unpredictability exert unique influence on life history outcomes at different developmental stages. For example, environmental unpredictability in childhood, but not environmental harshness, is uniquely associated with perpetration of intimate partner violence by both men and women (Barbaro & Shackelford Reference Barbaro and Shackelford2016) and criminal behavior in young adulthood (Simpson et al. Reference Simpson, Griskevicius, Kuo, Sung and Colling2012). Other research has demonstrated that environmental harshness in adolescent years exerts unique influence on deviant behavior in adolescence, such as drug and alcohol use (Brumbach et al. Reference Brumbach, Figueredo and Ellis2009). Environmental harshness and environmental unpredictability may therefore uniquely predict aspects of life history strategies, including aggressive behavior, over the life span.
The CLASH model does not readily accommodate, or discuss, the possibility that the dimensions of environmental risk may exert differential influence on the outcomes of interest. Conflating environmental harshness and environmental unpredictability into a single construct of environmental stress is not warranted, and may bias the results of research guided by the CLASH model. Ellis et al. (Reference Ellis, Figueredo, Brumbach and Schlomer2009) detail the properties of environmental risk dimensions and note how each dimension is associated with external threats to morbidity and mortality at various stages of development. Life history theorists, moreover, have suggested that environmental harshness and unpredictability may not exert equal influence on life history strategies (Roff Reference Roff2002), such that the effects of environmental unpredictability might be smaller than the effects of environmental harshness (Del Giudice et al. Reference Del Giudice, Gangestad, Kaplan and Buss2015).
Third, the construct of environmental unpredictability is not presented accurately in the target article. The authors assert that environmental unpredictability “refers to the uncertainty of future outcomes.” This definition is incorrect. Environmental unpredictability reflects the extent to which ecological factors produce variation in external morbidity and mortality threats (Ellis et al. Reference Ellis, Figueredo, Brumbach and Schlomer2009). Hypotheses derived from the CLASH model, therefore, rest on a faulty operationalization of environmental unpredictability.
The misunderstandings of life history theory embodied by the CLASH model, and the relationship between ecology and behavioral outcomes, are further evident in a primary proposition of the model. The authors suggest that greater seasonal variation in temperature should facilitate slower life history strategies, because seasonal variation necessitates that individuals “plan and prepare for the next season.” However, life history theory specifies how ecology influences external threats to morbidity and mortality (e.g., increased pathogen load in the environment), which then influence the adoption of life history strategies. Because the CLASH model does not specify which external morbidity and mortality threats are affected by temperature, it is not clear how temperature is hypothesized to impact human life history strategies. For instance, temperature variation might cause greater variation in specific sources of external morbidity and mortality in northern environments. During colder winter months, resource availability is lower. In the hotter summer months, pathogen load and disease prevalence are greater. Greater temperature variation, then, produces greater variation in particular external threats to morbidity and mortality. It could alternatively be argued that temperature variation could lead to fast, rather than slow life history strategies.
Rather than providing an “extension” of life history theory, the CLASH model for aggression is founded on misunderstandings of life history theory. The CLASH model incorrectly specifies how environmental harshness and environmental unpredictability exert influence on life history strategies. Future applications of the CLASH model should recognize that dimensions of environmental risk are, in fact, unique theoretical constructs.
Life history theory is a theoretical framework for addressing how and why organisms, including humans, allocate resources to conflicting life tasks (Del Giudice et al. Reference Del Giudice, Gangestad, Kaplan and Buss2015; Roff Reference Roff2002; Sterns Reference Sterns1992). Life history theory is complex and rich in its predictions but is often oversimplified when applied to human psychology and behavior. Van Lange et al. (2016) also present an oversimplification of life history theory as the foundation for the CLASH model proposed to explain aggression and violence across the world. In particular, the model incorrectly specifies how environmental harshness and unpredictability affect life history strategies and behavioral outcomes. Here, we discuss three misunderstandings of life history theory expressed in the target article.
First, the CLASH model appears to conflate the constructs of environmental harshness and environmental unpredictability into a single predictor of individual variation in life history outcomes (e.g., aggression). The model correctly identifies that both environmental harshness and environmental unpredictability function to regulate life history strategies. The model incorrectly implies, however, that harshness and unpredictability are dependent constructs. Ellis et al.'s. (Reference Ellis, Figueredo, Brumbach and Schlomer2009) dimensions of environmental risk – harshness and unpredictability – are independent constructs that exert unique influences on individual variation in life history strategies. The CLASH model of aggression specifies that environmental harshness and environmental unpredictability determine the overall stress of the environment. In turn, this overall environmental stress is predictive of aggression. Environmental harshness and environmental unpredictability can be positively associated, such that a given environment can be harsh and unpredictable. The CLASH model, however, does not account for, or acknowledge, that (1) dimensions of environmental risk can be inversely associated (e.g., a predictable but harsh environment), and (2) environments characterized as harsh or unpredictable are not hypothesized to be exclusively associated with fast life histories. Ellis et al. (Reference Ellis, Figueredo, Brumbach and Schlomer2009) specify the conditions under which high environmental harshness and greater environmental unpredictability facilitate the adoption of either fast or slow life history strategies (pp. 218, 230). Whether environmental harshness or environmental unpredictability is associated with faster life history strategies is determined by age-specific rates of morbidity and mortality. Depending on whether external threats resulting from ecological factors exert greater influence on (or variation of) juvenile or adult morbidity and mortality rates, environmental harshness and unpredictability can facilitate faster or slower life histories. The CLASH model does not specify whether ecological conditions of temperature affect threats to juvenile or adult morbidity and mortality, or both. And neither does the CLASH model specify whether or how average temperature or variation in temperature differentially affect juvenile and adult morbidity and mortality.
Second, the CLASH model assumes that environmental harshness and environmental unpredictability exert similar and equal influence on specific life history outcomes over the life span. However, research has demonstrated that environmental harshness and unpredictability exert unique influence on life history outcomes at different developmental stages. For example, environmental unpredictability in childhood, but not environmental harshness, is uniquely associated with perpetration of intimate partner violence by both men and women (Barbaro & Shackelford Reference Barbaro and Shackelford2016) and criminal behavior in young adulthood (Simpson et al. Reference Simpson, Griskevicius, Kuo, Sung and Colling2012). Other research has demonstrated that environmental harshness in adolescent years exerts unique influence on deviant behavior in adolescence, such as drug and alcohol use (Brumbach et al. Reference Brumbach, Figueredo and Ellis2009). Environmental harshness and environmental unpredictability may therefore uniquely predict aspects of life history strategies, including aggressive behavior, over the life span.
The CLASH model does not readily accommodate, or discuss, the possibility that the dimensions of environmental risk may exert differential influence on the outcomes of interest. Conflating environmental harshness and environmental unpredictability into a single construct of environmental stress is not warranted, and may bias the results of research guided by the CLASH model. Ellis et al. (Reference Ellis, Figueredo, Brumbach and Schlomer2009) detail the properties of environmental risk dimensions and note how each dimension is associated with external threats to morbidity and mortality at various stages of development. Life history theorists, moreover, have suggested that environmental harshness and unpredictability may not exert equal influence on life history strategies (Roff Reference Roff2002), such that the effects of environmental unpredictability might be smaller than the effects of environmental harshness (Del Giudice et al. Reference Del Giudice, Gangestad, Kaplan and Buss2015).
Third, the construct of environmental unpredictability is not presented accurately in the target article. The authors assert that environmental unpredictability “refers to the uncertainty of future outcomes.” This definition is incorrect. Environmental unpredictability reflects the extent to which ecological factors produce variation in external morbidity and mortality threats (Ellis et al. Reference Ellis, Figueredo, Brumbach and Schlomer2009). Hypotheses derived from the CLASH model, therefore, rest on a faulty operationalization of environmental unpredictability.
The misunderstandings of life history theory embodied by the CLASH model, and the relationship between ecology and behavioral outcomes, are further evident in a primary proposition of the model. The authors suggest that greater seasonal variation in temperature should facilitate slower life history strategies, because seasonal variation necessitates that individuals “plan and prepare for the next season.” However, life history theory specifies how ecology influences external threats to morbidity and mortality (e.g., increased pathogen load in the environment), which then influence the adoption of life history strategies. Because the CLASH model does not specify which external morbidity and mortality threats are affected by temperature, it is not clear how temperature is hypothesized to impact human life history strategies. For instance, temperature variation might cause greater variation in specific sources of external morbidity and mortality in northern environments. During colder winter months, resource availability is lower. In the hotter summer months, pathogen load and disease prevalence are greater. Greater temperature variation, then, produces greater variation in particular external threats to morbidity and mortality. It could alternatively be argued that temperature variation could lead to fast, rather than slow life history strategies.
Rather than providing an “extension” of life history theory, the CLASH model for aggression is founded on misunderstandings of life history theory. The CLASH model incorrectly specifies how environmental harshness and environmental unpredictability exert influence on life history strategies. Future applications of the CLASH model should recognize that dimensions of environmental risk are, in fact, unique theoretical constructs.