Abstract: The external environment has traditionally been considered as the primary driver of animal life history (LH). Recent research suggests that animals' internal state is also involved, especially in forming LH behavioural phenotypes. The present study investigated how these two factors interact in formulating LH in humans. Based on a longitudinal sample of 1223 adolescents in nine countries, the results show that harsh and unpredictable environments and adverse internal states in childhood are each uniquely associated with fast LH behavioural profiles consisting of aggression, impulsivity, and risk-taking in adolescence. The external environment and internal state each strengthened the LH association of the other, but overall the external environment was more predictive of LH than was the internal state. These findings suggest that individuals rely on a multitude and consistency of sensory information in more decisively calibrating LH and behavioural strategies.
4. Discussion
Existing research has shown, in separate studies, that both external environments and internal body states affect animal LH. Investigating these two factors in the same individuals, recent studies have demonstrated unique LH predictions by the two predictors (e.g. [33]). Findings of the present study add to the literature by showing the two factors interact in reinforcing LH calibration in the same direction. Childhood harsh and unpredictable environments and adverse internal states were each uniquely associated with fast LH behavioural profiles in adolescence, and each factor strengthened the LH association of the other factor. Specifically, at higher, compared to lower, levels of internal adversity, external harshness and unpredictability was more predictive of fast LH. Similarly, adverse internal state was predictive of fast LH when the external environment was harsh and unpredictable but not when it was benign. These conditional predictions suggest that external environments and internal body states likely reinforce the same cues about mortality–morbidity in accelerating fast LH. In the present study, we obtained information about childhood environmental harshness and unpredictability and adverse internal state at approximately the same time, 3 years before measuring LH profiles, therefore establishing longitudinal, unconditional, and conditional associations with or predictions of LH from the external and internal predictors. Additionally, external environment was more strongly associated with fast LH than was internal state as evidenced both by the two main effects (β = 0.50 versus 0.15) and the four conditional associations (figure 2). External environment may be an underlying cause of both fast LH behavioural profiles and internal body states [1] and, when this common variance was statistically accounted for in the present study by integrating the mediation modelling of the relation of external environment to internal state, LH prediction by the internal but not the external predictor was attenuated. These comprehensive findings support the conclusion that external environment has a stronger impact in shaping LH than internal state. Nonetheless, the internal state is still predictive of a fast LH behavioural profile overall and when external harshness and unpredictability are severe rather than benign.
Traditional LH theory emphasizes the environment as the sole actor in activating and formulating animal LH. Specifically, the mortality-causing harshness and unpredictability, as well as resource conditions (which were not investigated in the present study and have rarely been studied by other human LH research [8]), cause physiological (e.g. endocrine and homeostasis) and psychological tuning (e.g. cognitive and behaviour) that is oriented either towards growth and development (slow LH) or mating and reproduction (fast LH). Recent research has introduced animal internal state as a complementary actor in forming LH strategies [13,22,67]. In the light of the present findings, external environment and internal body state should not be distinguished as categorically separate drivers of LH. The two can be regarded as representing different and unique sensory inputs. For humans, the external environment is sensorially processed mainly as visual, auditory, and tactile data, whereas internal state is processed mainly as interoceptive including visceroceptive information. Humans and other animals processing more and more consistent sensory information should calibrate LH more decisively than those having less or less consistent information. Such is the implication of the present findings about the significant ordinal interaction effect. In the human case, for example, a child who constantly feels pains in her stomach while witnessing adult neighbours falling ill to a strange parasite should be more readily set on a faster LH path compared to the one who receives only one but not both sensory inputs or who receives two inconsistent inputs by experiencing external mortality but internal homeostasis or vice-versa.
A proviso to the present findings and discussion is that, like most of the existing human LH literature (e.g. [68]), the external environment operationalized in the present study was mostly individually specific, representing a person's unique home and family environment rather than a larger ecology shared with other conspecifics and thus increasing seeming consistency between external and internal conditions in impacting LH. This is not necessarily a limitation of the present study but, rather, represents the reality of LH research on contemporary human participants. Because humans have long mastered the external environment [69], most ecology-wide variables, such as mortality, pandemic and even intraspecific conflict that are traditionally defined in the species-general LH literature as extrinsic risks beyond individuals' survival effort, operate exactly depending on modern human individuals’ survival abilities or, more precisely, resource conditions. Thus, they do not fully satisfy the definition of extrinsic risks [70]. Because of these difficulties, individual-level variables, such as chaos at home, family misfunctioning, and unpredictable life events, have been substituted as proxies of extrinsic risks in human LH research (e.g. [40]). Another related weakness is using multi-informant reporting to measure internal states, as well as external environments, because individual differences in interoception may potentially be correlated with LH (e.g. [71]) and may in general confound the effects of body conditions. However, cognitive assessment of both internal and external information may become integral parts of human LH calibration. An appreciation of these human-specific conditions is needed in applying species-general models to study human LH. The present study represents such an appreciation and one of the first attempts to our knowledge to investigate how external environments and internal states interact in calibrating human LH behavioural profiles.
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