Cousins getting married is one of the oldest and most timeless jokes in the world. We know there’s precedent in real life but we don’t have to be experts on the matter to know that something weird happens when you have babies with someone who shares some of your genes.
Not that long ago, cousins getting married was a common occurrence. You might want to take a look at your family tree because it’s likely that at some point, some of your ancestors shared the last name before marriage.
Related Story: Why Do We Procrastinate Sleep Even When We’re Tired?
Popular Science reports a genealogy study that revealed that the practice of marrying cousins was once pervasive. In fact, from 1650 to 1850 a given person was, on average, fourth cousins with their spouse. As the years passed, this trend stopped being so common. By the 1950s, married couples were, on average, seventh cousins. This is due to the evolution of methods of transportation, which allowed people to move to different places and, ultimately, getting to know people who were not their family.
While the stats of marrying cousins decreased as the world evolved, the study reports that consolidating money and power was a big factor in marriages, which is why this phenomenon still occurred even after the industrial revolution. Yaniv Erlich, the man who conducted the study, believes that people stopped marrying their cousins because of an evolving society who no longer considered this acceptable. The slow progression of women’s autonomy also played a big part. Nowadays, marriage between first cousins is illegal in 24 states.
The genetic problem that pops up when first cousins get married is due to the fact that the parents share 12.5 percent of their DNA. This is bad for genetics, since diversity is very important for healthy babies. If you inherit a bad gene from your mom, then your dad’s genes will work hard to make up for that.
Statistics report that 4 to 7 percent of the kids who’s parents are first cousins are estimated to have birth defects. While it’s not a big number, it becomes more problematic if these kids were to marry their first cousins, resulting in an even smaller pools of genes. The farther you marry, the more you reduce the odds of passing down birth defects to your kids.
While this may not seem like a big problem for the modern world, it’s a thing in some places with small populations. In Iceland, where the population is of 330,000 people, it’s common for citizens to fear marrying a distant cousin.