Intergenerational conflicts may help explain parental absence effects on reproductive timing: a model of age at first birth in humans

Abstract

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. Family members often help each other reproduce, meaning that parents and offspring may find themselves in competition over reproductive opportunities. In these intergenerational negotiations offspring will have less incentive to help the remaining parent rear future half-siblings relative to beginning reproduction themselves.Method. We illustrate this “intergenerational conflict hypothesis” with a formal game-theoretic model.Results. We show that when resources constrain reproductive opportunities within the family, parents will generally win reproductive conflicts with their offspring, i.e., they will produce more children of their own and therefore delay existing offsprings’ reproduction. This is due to the asymmetric relatedness between grandparents and grandchildren (r = .25), compared to siblings (r = 0.5), resulting in greater incentives for older siblings to help rear younger siblings than for grandparents to help rear grandchildren. 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.

The impact of kin availability, parental religiosity, and nativity on fertility differentials in the late 19th-century United States

“…If these males contributed significant familial and economic help to the family, the childrearing assistance provided by older females may have been offset. 4 Additional possibilities include the potential for greater conflict or competition for resources in larger households, which has been shown to be relevant to childbearing in other contexts (e.g., Flinn 1989; Strassmann 2011, Moya and Sear 2014), and the potential role of duration of marriage and its relationship to stopping behavior. Although we have no precise measurement in the data, women with more co-resident females age 11 and older likely had longer marriages than women without co-resident older females.…”

Does grandparental help mediate the relationship between kin presence and fertility?

“…For instance, a woman of reproductive age may provide help to her kin, acting as helper-at-the-nest (Kramer 2005); resulting in close kin more effectively reproducing, but the woman herself suffering a reduction in fertility. This may occur because of reproductive competition between generations (Cant and Johnstone 2008;Moya and Sear 2014).…”

Marital Dissolution and Child Educational Outcomes in San Borja, Bolivia

“…In contrast, evidence from the matrilineal Khasi of northeast India has shown that kin, particularly mothers, may contribute to good marital choices and therefore reduce the likelihood of divorce (Leonetti et al 2007). Kin availability has been shown to have effects on many aspects of reproductive success including child survivorship (Borgerhoff Mulder 2007;Sear and Mace 2008;Sear et al 2002), age at marriage (Moya et al n.d.;Moya and Sear 2014), and reproductive rates (Sear and Coall 2011;Snopkowski and Sear 2013) and likely plays a role in marital dissolution as well.…”