CLASH proposes that warm and less variable climates are harsh and unpredictable, and that these conditions shift individuals toward faster life histories involving a here-and-now orientation, which sparks aggression and violence. These relationships can result from a variety of adaptive processes, including development, contextual plasticity, and social learning. Here, we focus on development. Specifically, we expand CLASH by considering two distinct, but mutually compatible, evolutionary accounts of the shift toward fast life histories: external and internal predictive adaptive responses (PARs) (Nettle et al. Reference Nettle, Frankenhuis and Rickard2013; Rickard et al. Reference Rickard, Frankenhuis and Nettle2014).
The external PAR account proposes that fast life histories evolved in response to factors in the external environment (e.g., warm, variable climates) that forecasted future environmental conditions that reduce life expectancy (e.g., famine) (Belsky et al. Reference Belsky, Steinberg and Draper1991). The internal PAR account, in contrast, proposes that fast life histories evolved in response to factors that directly reduced life expectancy (e.g., nutritional deprivation) by accelerating somatic aging (e.g., impairing cell repair), irrespective of future environmental conditions (Nettle et al. Reference Nettle, Frankenhuis and Rickard2013; Rickard et al. Reference Rickard, Frankenhuis and Nettle2014).
Mathematical modeling indicates that external and internal PARs evolve under different conditions (Nettle et al. Reference Nettle, Frankenhuis and Rickard2013; Rickard et al. Reference Rickard, Frankenhuis and Nettle2014). External PARs require that environmental conditions are stable over individuals' lifetimes (e.g., if there is drought today, there will likely be drought next year). Internal PARs do not require such stability, but rather require individuals' somatic conditions to be stable over their lifetimes (e.g., if my body is in a poor state today, it will be likely in a poor state next year). In an environment that is completely unpredictable, external PARs cannot evolve; however, internal PARs can, if earlier somatic states are correlated with later somatic states. Such somatic autocorrelation is well known to exist in humans (Blackburn et al. Reference Blackburn, Epel and Lin2015).
The internal and external PAR accounts often make the same predictions about the effects of environment on life history and hence, if integrated with CLASH, about the distributions of aggression and violence. For example, if warm and variable climates are harsh and unpredictable (as CLASH proposes), both accounts predict that such climates produce fast life histories, albeit for different reasons. The external PAR account posits this because it supposes that earlier environmental adversity predicts later environmental adversity. The internal account posits it because it supposes that earlier adversity accelerates somatic aging, irrespective of future environmental conditions.
Despite partial overlap in predictions, the internal and external PAR accounts make different predictions about individuals or populations of the same chronological age in two cases: (1) when they have the same somatic age, but have been exposed to different environmental conditions, and (2) when they have been exposed to the same environmental conditions, but vary in their somatic age. Here, we focus on the second case.
Individuals or populations that have been exposed to the same environmental conditions (e.g., drought) may vary in their somatic quality (e.g., tissue damage) for many different reasons, including genetic mutations, developmental noise, and chance events (e.g., idiosyncratic exposure to disease). The internal PAR account predicts that individuals or populations that have incurred higher levels of somatic damage (e.g., telomere attrition) will develop faster life histories than those within the same environment that have incurred lower levels of damage. This prediction does not follow from the external PAR account, as all people in this environment will be equally likely to suffer from death and disability caused by future external conditions.
Consistent with the internal PAR account, British data indicate that girls who experienced chronic disease in childhood develop faster life histories, as indicated by earlier timing of first reproduction, even though chronic disease was not correlated with other measures of environmental stress (e.g., father absence, parental socioeconomic status) (Waynforth Reference Waynforth2012). Similarly, Danish data indicate that low birth weight (a marker for somatic quality) predicts lower levels of trust in adulthood, even after controlling for multiple indicators of childhood family environment (e.g., mother's education and income). A possible explanation is that low birth weight predicts small size and physical vulnerability later in life, increasing the risk of being socially exploited (Petersen & Aarøe Reference Petersen and Aarøe2015).
The internal PAR account predicts that individuals who have incurred higher levels of somatic damage are more here-and-now oriented. Interestingly, in European starlings, individuals with greater developmental telomere attrition show higher impulsivity in their foraging decisions (Bateson et al. Reference Bateson, Brilot, Gillespie, Monaghan and Nettle2015). According to CLASH, a stronger here-and-now orientation increases aggression and violence. A caveat here is that adults who are physically small (e.g., because of low birth weight or disease in childhood) are less likely to be successful when engaging in aggression and violence (Sell et al. Reference Sell, Tooby and Cosmides2009). Therefore, tests of the internal PAR account should control for current somatic factors (e.g., body size) that affect the likelihood of winning fights.
Generally, the internal PAR account predicts that somatic age mediates the link between exposures to harsh, unpredictable environments and faster life histories. Testing this mediation would require measures of somatic aging (e.g., telomere attrition, oxidative stress), environmental stress (e.g., socioeconomic status), and aggression and violence of the same individuals. The external PAR account predicts that higher environmental stress predicts greater aggression and violence. The internal PAR account predicts this relationship as well (as environmental stress causes somatic damage), and additionally predicts that (1) higher somatic age (for a given chronological age) is associated with greater aggression and violence, and (2) the relationship between environmental stress and aggression and violence is reduced when somatic age is included as a mediating variable.
Of course, the internal and external PAR accounts are not mutually exclusive. It is possible that individuals predict both their future environment and their future somatic states and tailor their life histories accordingly. However, as these two accounts are distinct and make different predictions about life history in certain circumstances, CLASH could benefit from incorporating both.
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CLASH proposes that warm and less variable climates are harsh and unpredictable, and that these conditions shift individuals toward faster life histories involving a here-and-now orientation, which sparks aggression and violence. These relationships can result from a variety of adaptive processes, including development, contextual plasticity, and social learning. Here, we focus on development. Specifically, we expand CLASH by considering two distinct, but mutually compatible, evolutionary accounts of the shift toward fast life histories: external and internal predictive adaptive responses (PARs) (Nettle et al. Reference Nettle, Frankenhuis and Rickard2013; Rickard et al. Reference Rickard, Frankenhuis and Nettle2014).
The external PAR account proposes that fast life histories evolved in response to factors in the external environment (e.g., warm, variable climates) that forecasted future environmental conditions that reduce life expectancy (e.g., famine) (Belsky et al. Reference Belsky, Steinberg and Draper1991). The internal PAR account, in contrast, proposes that fast life histories evolved in response to factors that directly reduced life expectancy (e.g., nutritional deprivation) by accelerating somatic aging (e.g., impairing cell repair), irrespective of future environmental conditions (Nettle et al. Reference Nettle, Frankenhuis and Rickard2013; Rickard et al. Reference Rickard, Frankenhuis and Nettle2014).
Mathematical modeling indicates that external and internal PARs evolve under different conditions (Nettle et al. Reference Nettle, Frankenhuis and Rickard2013; Rickard et al. Reference Rickard, Frankenhuis and Nettle2014). External PARs require that environmental conditions are stable over individuals' lifetimes (e.g., if there is drought today, there will likely be drought next year). Internal PARs do not require such stability, but rather require individuals' somatic conditions to be stable over their lifetimes (e.g., if my body is in a poor state today, it will be likely in a poor state next year). In an environment that is completely unpredictable, external PARs cannot evolve; however, internal PARs can, if earlier somatic states are correlated with later somatic states. Such somatic autocorrelation is well known to exist in humans (Blackburn et al. Reference Blackburn, Epel and Lin2015).
The internal and external PAR accounts often make the same predictions about the effects of environment on life history and hence, if integrated with CLASH, about the distributions of aggression and violence. For example, if warm and variable climates are harsh and unpredictable (as CLASH proposes), both accounts predict that such climates produce fast life histories, albeit for different reasons. The external PAR account posits this because it supposes that earlier environmental adversity predicts later environmental adversity. The internal account posits it because it supposes that earlier adversity accelerates somatic aging, irrespective of future environmental conditions.
Despite partial overlap in predictions, the internal and external PAR accounts make different predictions about individuals or populations of the same chronological age in two cases: (1) when they have the same somatic age, but have been exposed to different environmental conditions, and (2) when they have been exposed to the same environmental conditions, but vary in their somatic age. Here, we focus on the second case.
Individuals or populations that have been exposed to the same environmental conditions (e.g., drought) may vary in their somatic quality (e.g., tissue damage) for many different reasons, including genetic mutations, developmental noise, and chance events (e.g., idiosyncratic exposure to disease). The internal PAR account predicts that individuals or populations that have incurred higher levels of somatic damage (e.g., telomere attrition) will develop faster life histories than those within the same environment that have incurred lower levels of damage. This prediction does not follow from the external PAR account, as all people in this environment will be equally likely to suffer from death and disability caused by future external conditions.
Consistent with the internal PAR account, British data indicate that girls who experienced chronic disease in childhood develop faster life histories, as indicated by earlier timing of first reproduction, even though chronic disease was not correlated with other measures of environmental stress (e.g., father absence, parental socioeconomic status) (Waynforth Reference Waynforth2012). Similarly, Danish data indicate that low birth weight (a marker for somatic quality) predicts lower levels of trust in adulthood, even after controlling for multiple indicators of childhood family environment (e.g., mother's education and income). A possible explanation is that low birth weight predicts small size and physical vulnerability later in life, increasing the risk of being socially exploited (Petersen & Aarøe Reference Petersen and Aarøe2015).
The internal PAR account predicts that individuals who have incurred higher levels of somatic damage are more here-and-now oriented. Interestingly, in European starlings, individuals with greater developmental telomere attrition show higher impulsivity in their foraging decisions (Bateson et al. Reference Bateson, Brilot, Gillespie, Monaghan and Nettle2015). According to CLASH, a stronger here-and-now orientation increases aggression and violence. A caveat here is that adults who are physically small (e.g., because of low birth weight or disease in childhood) are less likely to be successful when engaging in aggression and violence (Sell et al. Reference Sell, Tooby and Cosmides2009). Therefore, tests of the internal PAR account should control for current somatic factors (e.g., body size) that affect the likelihood of winning fights.
Generally, the internal PAR account predicts that somatic age mediates the link between exposures to harsh, unpredictable environments and faster life histories. Testing this mediation would require measures of somatic aging (e.g., telomere attrition, oxidative stress), environmental stress (e.g., socioeconomic status), and aggression and violence of the same individuals. The external PAR account predicts that higher environmental stress predicts greater aggression and violence. The internal PAR account predicts this relationship as well (as environmental stress causes somatic damage), and additionally predicts that (1) higher somatic age (for a given chronological age) is associated with greater aggression and violence, and (2) the relationship between environmental stress and aggression and violence is reduced when somatic age is included as a mediating variable.
Of course, the internal and external PAR accounts are not mutually exclusive. It is possible that individuals predict both their future environment and their future somatic states and tailor their life histories accordingly. However, as these two accounts are distinct and make different predictions about life history in certain circumstances, CLASH could benefit from incorporating both.