Robin Fox believes that 80% of all marriages in all of history were between second cousins or closer relatives, but that has been changing rapidly since the advent of industrialization. Today, only about 10% of marriages are consanguineous. Little of the change is due to legal prohibitions. Marrying a first cousin is only banned “in China and Taiwan, North Korea, South Korea, the Philippines and 24 of the 50 United States” according to Wikipedia. More likely, cultural changes have caused the legal changes in the few places where it is now banned. A new study gives some additional clues:
Before the Industrial Revolution …you might have ended up married to a fourth cousin. People didn’t travel far to find a spouse, and the closer you were to home, the more likely it was you’d marry within your family. Then, in the late 19th century, something changed, and people stopped marrying their cousins. It has been conventional wisdom that Europeans and North Americans married more outside their families as geographic dispersal ramped up between 1825 and 1875, with the advent of mass railroad travel. But over the same period, the genetic relatedness of many couples actually increased. It wasn’t until after 1875 that partners started to become less and less related.
This 50-year lag might indicate that shifts in social norms played a bigger role than geographic mobility in getting people to wed outside their bloodline. It’s also just one example of the insights that can be gleaned from the world’s largest, scientifically-vetted family tree, presented in a study published on Thursday in Science.
The Economist magazine argues that cousin marriage will have to drop when fertility rates drop due to the mathematical probabilities:
cousin marriage is doomed, if only for demographic reasons. In many countries where it is common, birth rates are plunging. In the early 1980s Pakistan’s fertility rate was 6.4 (meaning that a woman could expect to have that many children during her childbearing years). That number is now thought to have come down to 3.4, and UN demographers expect it to fall to 2.4 by the early 2040s. In Iran, the fertility rate has crashed from 6.5 in the early 1980s to just 1.6.
Two academics, Bilal Barakat and Stuart Gietel-Basten, point out that when women usually have five surviving children, a woman can expect to have 25 male cousins. When the average number of children falls to two, that same woman will have just three male cousins, some or all of whom might be younger than she is, and thus ineligible as marriage partners.
In addition to greater access to more diverse marriage partners due to urbanization and better transportation, and smaller families limiting the possibility of marrying within the family, a third factor is the increasing cosmopolitanism of modern life. People used to be much more tribalistic and that extended towards their attitudes towards marriage. We don’t have many measures of how much people disliked marrying outsiders who are not just like their family except for in the case of inter-racial marriage. Gallop has been tracking that for over a half century and says that it shows “one of the largest shifts of public opinion in Gallup history” Inter-racial marriage was taboo for all but 4% of Americans in 1958 when Gallop began polling about it. It was so taboo it was illegal in 19 states until 1967, when the U.S. Supreme Court ruled that marriage across racial lines was legal. Despite legalization and the civil rights movement, by 1995, 55% of nonblacks still did not approve of inter-racial marriage. That number has shrunken markedly since then, and by 2013, only 16% of non-black Americans did not approve of inter-racial marriage, but that still represented about 50 million Americans who were uncomfortable with marriage between people who were racially different from family.
This map from Matt Zang shows estimates of co-sanguineous marriage (inbreeding) around the globe: Yellow = <1%; Pink = 1%-10%; Green = 10%-50%