The expendable male hypothesis

Abstract

Matriliny is a system of kinship in which descent and inheritance are conferred along the female line. The theoretically influential concept of the matrilineal puzzle posits that matriliny poses special problems for understanding men's roles in matrilineal societies. Ethnographic work describes the puzzle as the tension experienced by men between the desire to exert control over their natal kin (i.e. the lineage to which they belong) and over their affinal kin (i.e. their spouses and their biological children). Evolutionary work frames the paradox as one resulting from a man investing in his nieces and nephews at the expense of his own biological offspring. In both cases, the rationale for the puzzle rests on two fundamental assumptions: (i) that men are in positions of authority over women and over resources; and (ii) that men are interested in the outcomes of parenting. In this paper, we posit a novel hypothesis that suggests that certain ecological conditions render men expendable within local kinship configurations, nullifying the above assumptions. This arises when (i) women, without significant assistance from men, are capable of meeting the subsistence needs of their families; and (ii) men have little to gain from parental investment in children. We conclude that the expendable male hypothesis may explain the evolution of matriliny in numerous cases, and by noting that female-centred approaches that call into doubt assumptions inherent to male-centred models of kinship are justified in evolutionary perspective. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

Keywords: gender; kinship; mating systems; matrifocality; matriliny; parental investment.

Figure 1.

Investments in daughters versus sons. (a) Trivers–Willard. The fitness ceiling is lower for females than for males. According to the Trivers–Willard hypothesis, under certain circumstances, this leads to steeper marginal gains to fitness from resources to males. In such cases, poorly resourced parents may benefit from investing in daughters whereas richly resourced parents may benefit from investing in sons (redrawn from Cronk [99]). (b) Matriliny-as-daughter-biased investment (MDBI). The MDBI hypothesis incorporates the differential effects of resources on male and female fitness shown in (a) and the effect of paternity certainty to predict when parents will invest in daughters (matriliny) versus sons (patriliny). The shaded area shows that parents will invest in daughters whenever the additional benefit of investing in sons (Bs/Bd) does not compensate for risk of non-paternity in sons' offspring (redrawn from Mattison [49] based on Holden et al. [63]). (Online version in colour.)