To test the theory that the intergenerational transfer of wealth plays a pivotal role in the evolution of human sibling relationships, Dr Mhairi Gibson and Dr Eshetu Gurmu studied the Arsi Oromo agro-pastoralist society in present-day Ethiopia, where recent changes in land tenure policy have altered the nature of transfers of resources from parents to offspring.
They found that the extent to which adult siblings competed or co-operated with each other in marriage and reproduction was strongly influenced by the presence or absence of heritable wealth.
In households where land was inherited, having a greater number of elder brothers reduced a man’s agricultural productivity, marriage opportunities and reproductive success (surviving offspring), as resources diminished and competition increased with each additional male sibling.
Conversely, where land was not inherited but distributed equally by the government, older siblings did not have a competitive effect and in some instances could be beneficial. Older sisters’ bridewealth (payment made by a groom or his kin to the kin of the bride) was used to obtain higher status marriages for younger brothers, and in doing so, offset most of the competitive effects of older brothers.
Dr Gibson said: “Since the end of government land redistribution programs in the early 1990s, the Arsi Oromo have experienced greater competition between brothers for high quality land and mates. Despite current government legislation to encourage the equal division of heritable resources among children, Arsi Oromo parents have adopted a pattern of wealth inheritance which favours elder sons, who obtain better quality land and higher bridewealth payments.
“This may also be having an effect on family sizes: among the Arsi Oromo, contraceptive use has increased from less than one per cent to over 19 per cent between 1999 and 2009. Among the first to adopt the use of modern contraception are those farmers who have inherited the smallest parcels of land.”
These findings contribute to growing evidence that competition for heritable resources, such as land, cattle and titles, represents an important determinant of the nature of sibling relationships, and also parental reproductive decision-making. Recent changes in wealth transfers which have driven sibling competition may be contributing to an increased desire for smaller family sizes.
‘Land inheritance establishes sibling competition for marriage and reproduction in rural Ethiopia’ by Mhairi A. Gibson and Eshetu Gurmu in PNAS