Marriage between cousins, considered a taboo in many cultures, may benefit their parents, scientists have discovered. Their findings help explain why the practice remains embedded in many cultures around the world, despite being frowned upon in others.

The practice of arranged marriages is ancient, dating as far back as marriage itself. Letting parents determine the best partners possible for their children, based on strategic considerations, has been central to many cultures, at many points throughout history.

The most common form of marriage arrangement across cultures is cross-cousin marriage – marriage between cousins born to two opposite-sex siblings. In contrast, marriage between parallel cousins (born to same-sex siblings) is often more stigmatised.

Although there has been a lot of research conducted on the subject of cross-cousin marriage, the origins and evolution of the practice remain unclear. Scientists are not sure why it is often exempted from social taboo.

In a study published in PNAS, a team has investigated the consequences of consanguineous marriage among the Yanomamö, a tribal society in the Amazon rainforest.

Marriages in Yanomamö culture

The team examined genealogical data from the Yanomamö to find out who within the tribe benefited from cross-cousin marriages and inbreeding.

The Yanomamö practice prescriptive bilateral cross-cousin marriage. In other words, males are expected to marry the daughters of their parents' opposite-sex siblings. The scientists used a statistical model to analyse genealogical data and find out how people's reproductive success was affected by the practice.

"This study involved interviewing over 5,000 individuals over some 30 years. Then I literally counted numbers of offspring and calculated how each child was related to both parents", lead author Napoleon Chagnon, from the University of Missouri, told IBTimes UK.

With his colleagues, he observed that close-kin marriage and inbreeding slightly reduces the fertility of both husbands and wives. Furthermore, their own children suffer from an inbreeding depression – a reduced ability to survive and perpetuate their genetic material. "Both sons and daughters of parents who were more closely related had significantly fewer total offspring of their own", the authors explain.

But these consanguineous partnerships appear to be beneficial for the parents of the cousins who got married. Indeed, the statistical model finds that they have a greater number of grandchildren as a result – something which may sound paradoxical given that their children who marry their cousins are less fertile.

However, the authors explain that in Yanomamö culture, exchanging daughters with relatives enable parents to obtain more brides for their sons, and this may generate a greater total number of grandchildren. There also seems to be benefits for the brothers of women who get married with their cousins, although the reason for this is unclear.

These arrange unions reveal conflicts between parents and their offspring, who view marriage with their cousins as less desirable. "One of the hypotheses we considered is that the fitness interests of grandparents are not the same as those of their children", Chagnon concluded.