Cooperation and conflict in the human family
Despite the crucial importance of Hamilton's (1964) kin selection theory in evolutionary behavioral biology, psychological studies of family relationships have been relatively slow to incorporate a Darwinian perspective. One practical reason may be that existing evolutionary models of animal families, such as the honest signaling models, are applicable only if all family members fall into the same class in terms of age, sex, or health. The animal models are thus of limited use for investigating human families, in which the relative age of the child, as a corollary of birth order, may have played a pivotal role in shaping evolved family psychology. My dissertation has two main objectives: 1) to construct evolutionary mathematical models of family interactions that fully take into account the role of reproductive value and hence can be directly applied to human families; 2) to characterize the design features of evolved psychological mechanisms of human kinship by empirically testing a priori predictions derived from the models. I first examine how parents are expected to allocate their limited resources among offspring of differing ages. I show that the optimal strategy that serves parental interests is to bias parental resources toward the older offspring (chapter 2). I then empirically test the predictions derived from the first study, in comparison with previous evolutionary hypotheses of parental favoritism. The empirical results confirmed the predictions derived from the first study: in hypothetical allocation tasks, participants allocated more tangible resources toward older children (chapter 3). Next, I investigate how intrafamilial conflict over the allocation of parental resources occur when each family member (a parent, its senior offspring, and its junior offspring) are allowed to differ in age. The results gained in this study may require a substantial revision of Trivers' (1974) classical theory of parent-offspring conflict. Moreover, it will open a fruitful avenue for inferring the adaptive design of psychological mechanisms dealing with sibling relationships (chapter 4). I then show that evolutionary insights can be also applied to the psychological study of distant kin relationships such as cousins (chapter 5).