A few years ago I gave a sermon at my (very liberal) church asking the question: “What determines the limit to our tolerance?” After the service, one member of the congregation took me to task for suggesting that humans are biologically hardwired to avoid consanguineous sexual relationships (those with close biological relations). Incest, he argued, could be a wonderful thing if society would only learned to tolerate it.
Henry died last month and I have to admit to feeling slightly ashamed that I never was prepared to hear him out on this topic. And so today, in his honor, I thought I would take on the topic of the economics behind consanguineous marriage.
Imagine for a moment that your adult children are home for Easter weekend. During a family dinner your daughter says, “Mom, Dad we have something to tell you. Brother and I are madly in love.” You respond, “That’s wonderful honey, when do we get to meet these special people?” After an awkward pause your son says, “No, you don’t understand. We are in love with each other. We have been dating since we came home for Christmas.”
On this one point, I do think my friend was wrong. Human biology is not well suited to being given this type of news and taking it well. I believe this because if my children did this I would have a very physical response.
Having said that, there are millions of people in the world today that are married to a close blood relation. Approximately 10% of the global population is married to someone who is either their second cousin or a closer relation.
In some regions of the world, Pakistan for example, more than 60% of marriages are of this sort.
The relationship between economic development and consanguineous marriage is a complex one. The wealthy nations of the world have very low rates of marriage between close relations -- generally at or less than 1% of all marriages. But even within those nations, some sub-populations continue to have high rates consanguineous marriage.
For example, at 33% of all marriages the Mennonite community in Kansas has a consanguinity rate much higher than many poor nations.
The observation that wealthy nations have low consanguinity rates does not mean that economic development in poorer nations has reduced the level of marriages within kin groups. In some regions, increases in wealth levels have been related to concurrent increases in the consanguinity rate.
The reason for this is that as nations' incomes grow populations become healthier and children are more likely to live to adulthood. In the early stages of development, families having more children who live to adulthood means that they have greater opportunities to arrange marriages between cousins etc., if that is the preference of society, simply because there are more cousins to choose from.
In later stages of development, however, reduced mortality in childhood and increased returns to education encourage families to have fewer children. When that happens consanguineous marriages become harder to arrange and the rate starts to decline.
The perfect example comes from the rapid industrialization that took place in Japan in the second half of the 20th Century. In just 20 years, the consanguinity rate in Tokyo fell from 4.6% (1961) to only 0.4% (1981).
In other regions, fertility rates have become so low that consanguineous marriage is impossible.
For example, the one child policy in China (in the communities in which the rule has been applied) has virtually eliminated the existence of both siblings and cousins, thus making rules prohibiting consanguineous marriage redundant.
Also, with industrialization the importance of land in determining family income declines as waged employment takes over family farming as the primary source of income. Reducing the importance of agricultural land in the economy should reduce consanguinity, especially if women cannot inherit their fathers' land.
This is as true today in India as it was in England in the time in which Jane Austen was writing her famous stories that featured young women being forced to marry their cousins so that they could continue to benefit from the family estate.
Finally, with development comes the importance of education in the workforce. Education tends to delay the age at which people marry, giving them an opportunity to choose their own marriage partner independent of family influence. Not surprisingly, even in countries with high rates of within-family marriage, individuals who are educated are much less likely to be married to a cousin, uncle, or sibling, etc.
These issues have important welfare implications. Recent evidence suggests that even after controlling for income per capita, people in countries with high rates of inbreeding have significantly lower levels of life expectancy.
Now let's get back to the idea that even the most liberal among us can be intolerant of the individual choices that do not directly affect us. I would like to say that personally I don’t have any issues with cousins marrying. And like many things, that tolerance stems from the fact that I have friends who are happily married first cousins.
They met at a family reunion, fell in love and decided to marry. After seeking advice from a genetics councilor who told them it was safe, they had three beautiful, and very healthy, children. They face discrimination for their decision to marry someone who is closely related to them, and to have children, but they don’t keep it secret either. Really, at the end of the day, why should they?
Mostafa Saadat (2011). “Association Between Healthy Life Expectancy At Birth And Consanguineous Marriages In 63 Countries.” Journal of Biosocial Science, 43: p.p. 475-480 doi:10.1017/S0021932011000034
For excellent references on consanguineous marriage, including international and regional comparisons, you might want to consult this resource here.