Abstract
The matrilineal Mosuo of southwest China live in large communal houses where brothers and sisters of three generations live together, and adult males walk to visit their wives only at night; hence males do not reside with their own offspring. This duolocal residence with ‘walking’ or ‘visiting’ marriage is described in only a handful of matrilineal peasant societies. Benefits to women of living with matrilineal kin, who cooperate with child-care, are clear. But why any kinship system can evolve where males invest more in their sister's offspring than their own is a puzzle for evolutionary anthropologists. Here, we present a new hypothesis for a matrilineal bias in male investment. We argue that, when household resources are communal, relatedness to the whole household matters more than relatedness to individual offspring. We use an inclusive fitness model to show that the more sisters (and other closely related females) co-reside, the more effort males should spend working on their sister's farm and less on their wife's farm. The model shows that paternity uncertainty may be a cause of lower overall work rates in males, but it is not likely to be the cause of a matrilineal bias. The bias in work effort towards working on their natal farm, and thus the duolocal residence and ‘visiting marriage’ system, can be understood as maximizing inclusive fitness in circumstances where female kin breed communally.
Highlights
Human families are very diverse, showing a wide range of residence patterns, be it among hunter–gatherers, farmers or others; among farmers, females usually but not always disperse at marriage [1]
We suggest a new hypothesis for a matrilineal bias in male investment, which is that working for sisters can evolve when female kin breed communally
Our results suggest that males in this system do not invest in matrilineal kin owing to high paternity uncertainty, nor are they being forced to invest in matrilineal kin against their reproductive interests
Summary
Human families are very diverse, showing a wide range of residence patterns, be it among hunter–gatherers, farmers or others; among farmers, females usually but not always disperse at marriage [1]. Sisters and adult daughters have their own rooms, where their husbands visit them at night; husbands do not eat in their wife’s household (except in rare 2 circumstances, for example, if the husband is helping the family with his labour during the planting season) [17] Males eat in their natal household but are expected either to visit their wives at night, or share another room in the house with the unmarried men and boys. Estimates of paternity uncertainty in human societies do not appear to be anywhere near high enough for this to be what is maintaining matrilineal social organization (one of the highest estimates being in the Himba who report only 17% extra-pair paternity [23]) Another model shows that matrilineal social organization is likely to arise only when males are polygynous, and when returns on resources conform to certain functions characteristic of more extensive systems [11]. That, whether or not females or males are mating polygamously, if female kin are breeding communally, males can maximize inclusive fitness by working predominantly for their natal household, which is the household to which they are most related
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