How Cousin Marriage and Polygyny Impair Modernisation

Cousin marriage is increasingly being legalized around the world, yet it remains illegal in most US states. But do American legislators have good reasons to outlaw it?

The most frequent concern regarding cousin marriage is biological. These risks have been grossly exaggerated. Sibling and parent-child incest are indeed dangerous. Close incest carries an increased risk that deleterious recessive genes will be expressed in offspring. But more distant kinship between parents may be advantageous. For example, as evolutionary biologist Patrick Bateson explains, “a woman with small jaws and small teeth who had a child by a man with big jaws and big teeth lays down trouble for her grandchildren, some of whom may inherit small jaws and big teeth. In a world without dentists, ill-fitting teeth were probably a serious cause of mortality.” Cousin marriage would make this mismatch less likely.

A very thorough study on the danger of cousin marriage concluded that the risk of genetic abnormalities in the resultant offspring is roughly the same as that for a 40-year old mother. Since it is not illegal for a 40-year-old woman to marry, why should it be illegal for cousins?

In his authoritative study of American attitudes towards cousin marriage, Martin Ottenheimer has argued that disapproval of the practice has more to do with American ideas of cultural superiority. In the nineteenth century, when most cousin marriage legislation was passed, the practice was perceived as a relic of a barbarous past. Anthropologist Lewis Henry Morgan argued that, before civilization, humans lived in promiscuous, incestuous hordes. Americans believed that a civilized country could not allow a practice so close to the incestuous past of our species.

We now know that Morgan’s theories were wildly speculative and mostly wrong. There was no promiscuous horde prior to civilization, and the biological risks of cousin marriage are low. Ottenheimer insists that the usual justifications for outlawing cousin marriage are very weak and therefore advocates the legalization of cousin marriage.

For many centuries, cousin marriage was also illegal in Western Europe. This was the Medieval Catholic church’s doing—probably with less than noble motives. The church authorized some cousin marriages, but charged significant amounts of money for the privilege. And, by forbidding cousin marriage, kinship groups were weakened; this meant fewer claimants to an inheritance, and any unclaimed property was administered by the church.

Nevertheless, the church prohibition has had a tremendously positive effect in modern times. That, at any rate, is the thesis of Joseph Henrich’s provocative new book The WEIRDest People in the World: How the West Became Psychologically Peculiar and Particularly Prosperous. Henrich claims that by forbidding cousin marriage, the church replaced kinship with other forms of social organization and this changed the way we think in the west. As people interacted more with non-blood relations, they became more modern. Society was organized along lines of merit, not just kin status. There was less nepotism. People became more cooperative, because they now cared about all human beings, not just their relatives. States replaced tribes. In Henrich’s analysis, the prohibition of cousin marriage had a snowball effect on a host of other transformations, allowing the west to rise to a position of world dominance over other cultures that practiced cousin marriage to a much greater extent.

Despite the huge amounts of data in Heinrich’s 600-page book, his thesis is tentative. But, if proven, it will raise questions about the wisdom of legalizing cousin marriage. Henrich has come up with a Kinship Intensity Index, which measures how important kinship is in any given society, and the prevalence of cousin marriage is a variable that increases that measure. Henrich has extensively documented an inverse correlation between kinship intensity and higher levels of education, cosmopolitan altruism, security, economic prosperity and the strength of democratic institutions. So, cousin marriage may pose no significant biological risk, but it does pose some societal risks.

Predictably, the US scores low on the Kinship Intensity Index. But it seems unlikely that legalizing cousin marriage would threaten democratic institutions or economic prosperity in the US. Few cousins want to get married, anyway. Legalizing cousin marriage would make those few people happy, and the social effects would be negligible. After all, according to Henrich’s research, Europe has an even lower Kinship Intensity Index, yet, in most European countries, cousin marriage is legal.

But, in those countries where cousin marriage is far more prevalent—especially in the Muslim world—legislators should consider introducing some restrictions. Cousin marriage is still too deeply enshrined in many countries, and legislators may encounter stiff resistance to outlawing it. But there may be other ways of weakening kinship ties, by targeting institutions that also enshrine the prominence of kinship.

Polygyny is one such institution. In polygynous societies, fewer men reproduce, which increases genetic group relatedness. By having various wives, men build extensive households that operate as tribes. Furthermore, polygyny increases the number of unmarried men, who have fewer opportunities to form families of their own, and have to stay in their natal homes, thus retaining their ties with their extended families.

Unlike cousin marriage, polygyny has concrete victims. Many studies suggest a strong relationship between polygyny and domestic violence. Women in polygynous societies also generally have fewer opportunities for development and are more vulnerable.

Despite its religious sanction in Islam, polygyny is being questioned by modernizers in some Muslim countries. It has already been banned in Turkey and Tunisia, and some other countries require consent from the first wife, or allow marriage contracts to prohibit polygyny. Contrary to Orientalist fantasies, Islam is not a monolithic petrified civilization, and there is plenty of room for dynamism and modernization. But, such modernization cannot be achieved unless kinship is weakened.

This weakening of kinship has already taken place in the west, in one of the great transformations of human history. Now the US can afford to relax its prohibitions on cousin marriage, to avoid punishing victimless crimes. But those countries that are still on the path to modernization need to enact a transformation similar to that achieved by the church in medieval Europe. They should begin by challenging polygyny, an institution that claims real victims and impairs societal modernization.