An intergenerational conflict model of age at first birth in humans

Background: Parental absences in childhood are often associated with accelerated reproductive maturity in humans. These results are counterintuitive for evolutionary social scientists because reductions in parental investment should be detrimental for offspring, but earlier reproduction is generally associated with higher fitness. In this paper we discuss a neglected hypothesis that early reproduction is often associated with parental absence because it decreases the average relatedness of a developing child to her future siblings. Method: We illustrate this “intergenerational conflict hypothesis”with a formal game-theoretic model. Results: . We show that parents will generally win reproductive conflicts with children when parents and children share limited household or kin resources, thus prioritizing their own reproduction and delaying offspring reproductive maturity. This is due to the asymmetric relatedness between grandparents and grandchildren (r=.25), compared to siblings (r=0.5) However, if a parent loses or replaces their partner, the conflict between the parent and offspring becomes symmetric since half siblings are as related to one another as grandparents are to grandchildren. This means that the offspring stand to gain more from earlier reproduction when their remaining parent would produce half, rather than full, siblings. We further show that if parents senesce in a way that decreases the quality of their infant relative to their offspring’s infant, the intergenerational conflict can shift to favor the younger generation, suggesting that it is primarily younger parents who should delay their offspring’s reproduction. Discuss. We use insights from this model to discuss the father absence literature and develop predictions about how the effects of parental absences should vary cross-culturally.