Land inheritance establishes sibling competition for marriage and reproduction in rural Ethiopia

Abstract

Intergenerational transfer of wealth has been proposed as playing a pivotal role in the evolution of human sibling relationships. Sibling rivalry is assumed to be more marked when offspring compete for limited heritable resources, which are crucial for reproductive success (e.g., land and livestock); whereas in the absence of heritable wealth, related siblings may cooperate. To date, comparative studies undertaken to support this evolutionary assumption have been confounded by other socioecological factors, which vary across populations, e.g., food sharing and intergroup conflict. In this article we explore effects of sibling competition and cooperation for agricultural resources, marriage, and reproduction in one contemporary Ethiopian agropastoralist society. Here recent changes in land tenure policy, altering transfers of land from parents to offspring, present a unique framework to test the importance of intergenerational transfers of wealth in driving sibling competition, while controlling for socioeconomic biases. In households where land is inherited, the number of elder brothers reduces a man's agricultural productivity, marriage, and reproductive success, as resources diminish and competition increases with each additional sibling. Where land is not inherited (for males receiving land directly from the government and all females) older siblings do not have a competitive effect and in some instances may be beneficial. This study has wider implications for the evolution of human family sizes. Recent changes in wealth transfers, which have driven sibling competition, may be contributing to an increased desire for smaller family sizes.

Parental investment and the optimization of human family size

“…Number of sisters, however, had a moderately positive effect on male fertility. Similar effects on the number of surviving offspring have been demonstrated on the Kenyan Kipsigis [33] and the Ethiopian Arsi Oromo [41]. Gillespie et al [34] found that large sibships reduced survival, but not fertility among survivors in eighteenth-to nineteenth-century Finland.…”

“…Supporting this explanation, Voland & Dunbar [36] found that negative effects of large family size were unique to landowning families in the Krummhö rn, while for peasants, offspring success was determined by other means. Gibson and Gurmu [41] also shows that number of older brothers holds negative associations with marital and reproductive success (number of surviving offspring) in rural Ethiopian farmers only when land quality had been determined by inheritance from parents. In otherwise comparable men who received land allocations from a government redistribution scheme, the existence of older brothers had no negative consequences.…”

“…'Fertility': no. of offspring born, except for Borgerhoff Mulder [33,69] and Gibson [41] [ 36] and were actually associated with higher fertility for males [28,30]. Predicted negative relationships between family size and child survival have been more successfully demonstrated in a number of contemporary African agriculturalist societies ( [31,32]; but see [33]) and historical European and American populations [34 -36].…”

Too many men: the violence problem?

“…Although the confidence bands are wide enough that a horizontal or upward-sloping regression line could be fitted, the line that best fits the data is negatively sloped. Details on individual data points may be found in [70][71][72][73][74][75][76][77][78][79][80][81][82]97,98].…”

Are wives and daughters disadvantaged in polygynous households? A case study of the Arsi Oromo of Ethiopia

“…Mean parity (live births) is 6.2 children (std dev 2.83). Mortality rates remain high but have been declining in recent decades (Gibson & Gurmu, 2011). Inheritance is patrilineal and husband and wife generally reside with the husband's patrilineage at marriage (patrilocal residence).…”