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Kissing Cousins: Why Haven’t Arranged Marriage Laws Reduced Human Genetic Diversity?

Credit: Elka Lesmono

Credit: Elka Lesmono

By Murray Cox

Many traditional communities, including our ancestors, have long enforced marriage between first cousins. Why hasn’t this had a negative impact on genetic diversity?

In the middle of the rainy season in 1975, cultural anthropologist Gregory Forth walked into the village of Rindi on the small island of Sumba in eastern Indonesia. For 2 years he studied the social rules, ethnography and environment of this remote community, including the cultural meaning of their distinctive house styles and megalithic monuments. He also recorded their intricate customs and rules around marriage.

Rules that dictate who we can and cannot marry were once more common than they are in western societies today. Our rules are now typically few: you cannot marry your siblings, but you can usually marry your cousin. (Although frowned on by many, including some religions, the only country in the western world to actually outlaw first cousin marriage is the United States. Even there, the rules vary widely: you can marry your first cousin in California but not in Mississippi). In contrast, the rules prescribing options for marriage partners in most traditional societies are far more ornate.

In Rindi, people follow a particularly complicated set of rules that date back centuries. A man must marry his first cousin, but only on his mother’s side. Communities are subdivided into clans, with a woman moving to the clan of her husband after marriage. Clans are further structured into “wife-giver” and “wife-taker” groups.

Generation after generation, women move in one direction around the network of clans, while bride wealth flows in the opposite direction. Marriages thus help to create stable, long-term alliances between different sections of the community.

Anthropologists have been studying intricate marriage systems like the one on Rindi for most of the past century. Their conclusion is that marriage rules play an important social role that secures long-term trade and support networks between communities. This, in turn, encourages collaboration over war, and provides a natural welfare system in times of need.

Marriage is also the primary institution leading to children, so it seems reasonable that marriage rules should strongly affect patterns of genetic diversity in the people who practise them.

Despite a century of anthropological study, few people have looked into the biological implications of marriage rules. Why does this matter? How are the social customs of Rindi relevant to an office worker in New York or Sydney?

The answer is that large cities are a modern invention. If you could look back just a few thousand years, all of our ancestors – in both Rindi and Sydney alike – lived in small communities of at most a few thousand people. These groups likely practised the same sorts of complex marriage rules that the community of Rindi still follow today.

This history, in turn, produced the patterns of DNA that we carry inside us. If we want to understand the structure and diversity of our genetic inheritance, we need to understand places like Rindi.

Determining the effects of marriage rules on genes seems like an obvious question, so why has it not already been answered? The reason comes down to two fundamental problems.

First, you need large amounts of genetic information to see the patterns made by marriage rules. Until recently, genetic data was limited and hard to collect, but high-throughput DNA screening technologies are now radically changing this landscape.

Second, you need a way to extract history from the genetic data, which is otherwise little more than strings of DNA bases. We do this through computer modelling, but the software and computer power needed to run these historical simulations is only just becoming available.

My research group, together with colleagues in Indonesia, Singapore and the United States, is starting to put the pieces of this puzzle together in research that has now been published in Molecular Biology and Evolution (tinyurl.com/p6utw52).

Because geneticists have not historically studied marriage rules, we first had to develop a computer program to explore how marriage systems affect genetic patterns in small traditional communities. When we modelled the marriage system in Rindi we discovered an interesting fact.

If people followed the marriage rules strictly, the community would experience a major decline in genetic diversity. However, if just one in five people did not follow the rule, then genetic diversity is almost the same as under random mating.

Simulations show that the declines in genetic diversity would occur on the nuclear chromosomes, but also sex-linked regions of the genome such as the X chromosome and mitochondrial DNA (which is passed down only from a woman to her children). Because all traditional communities follow some sort of marriage rules, the fact that adhering to them strictly produces such negative genetic outcomes was a surprise.

To understand what happens in the real world, our group then studied genetic diversity in Rindi by asking how well the marriage rules were actually followed. Although the people of Rindi say that they follow the rules strictly, their genes tell a very different story. From genetic patterns, it appears that the people of Rindi have historically followed their complicated marriage rules only about 50% of the time. This means that genetic diversity in Rindi is similar to what would be expected if people simply chose a marriage partner at random.

This is a good thing. In small communities – the only type that existed throughout most of human history – genetic diversity is easily lost, which in turn can lead to lower fertility, higher rates of genetic disorders and even the extinction of communities. Following the marriage rules strictly would only make these outcomes worse, especially for places like Rindi where the population is only a few thousand. Sometimes breaking the rules is the right thing to do.

An unanswered question is whether the rules are broken by choice or necessity. One possibility is that the community only weakly enforces the rules. However, in other places, such as Bali, strict compliance can be driven by strong sanctions against transgressors. This was often mediated through belief systems such as accusations of witchcraft, and not uncommonly led to the ultimate sanction, death.

More prosaic causes are also possible. In many cases there may not have been an appropriate partner to marry – a man’s mother might have had no brothers, or his uncle might have had no daughters. Men would then have had little choice but to break the rules.

Non-paternity, where the husband is not the biological father of a child, is another possible explanation. In western society, rates of non-paternity commonly reach 10%, but cheating is presumably universal and not restricted to the west.

Alternately, some men might simply have chosen to ignore the rule – perhaps to forge alliances between families in other ways.

An important point is that cultural behaviours such as marriage rules still affect us today. They can do so indirectly by affecting the genetic patterns of everyone alive right now. The diabetes of an office worker in Sydney is in part a reflection of who his ancestors chose to marry.

For many other people, the effects of marriage rules lie closer to home. Cultural practices of arranged marriage, often with first cousins, is common in many areas of South Asia and the Middle East. These have often been imported wholesale into many western countries with large immigrant populations.

Even in western societies, marriage practices that some might find strange today were once far more commonplace. Poet Edgar Alan Poe, physicist Albert Einstein and ex-Iraqi president Saddam Hussein all married their cousins. Perhaps ironically, so too did naturalist Charles Darwin, the composer of the theory of evolution.

When Gregory Forth walked into Rindi 40 years ago, it is unlikely that he fully realised what an interesting society he had stumbled upon. Today it is clear that the cultural practices he observed in Rindi have much wider biological repercussions. The DNA carried by all of us is the product of the marriage rules practised by our ancestors in the past.

Murray Cox is a Rutherford Fellow at the Institute of Fundamental Sciences, Massey University.