An analysis of European monarchs between the years 990 and 1800 has found that how inbred a king or queen is and how effective they were as a ruler are linked.
Throughout history, the European royal families have been quite partial to massive crowns and inbreeding. Royals through the continent would get strategically married to close relatives. This is ideal if your goal is to consolidate power, but not if you're also interested in producing healthy offspring and the avoidance of incest.
The results of inbreeding could be seen on the faces of the families, infamously in the distinctive Habsburg Jaw which was likely the product of generations of incest.
A new analysis from Sebastian Ottinger and Nico Voigtländer of UCLA has looked at the possible effect that this inbreeding has had on the reigns of the rulers, given that research has suggested that people born of incest tend to have lower intelligence.
In order to assess the performance of the 331 European monarchs studied, the researchers used previous analysis by US historian Adam Woods, who set about "grading" individual royals on their intelligence based on the writings of many other historians. This was combined with a second, more objective measure of performance: changes in area controlled by the rulers during their reigns. A larger area being acquired during a reign was seen as successful, and losing area the sign of a less effective reign.
To measure levels of inbreeding, the team not only looked at the parents of the ruler but at the inbreeding of their ancestors as well, as sourced from a genealogical database of European monarchs. Using Carlos II of Spain as an example – the individual they identified as having the highest coefficient of inbreeding – the team say he was "more inbred than offspring of siblings would be" despite his parents being "merely" uncle and niece.
"This points to an important feature of our setting: A sizable amount of the observed inbreeding is not the result of just one generation of consanguineous mating, but rather driven by a 'build up' of inbreeding over previous generations," the team wrote in their paper.
The researchers made sure to account for other factors, such as when a regent stood in for a child king or queen (the monarchy is such a wonderful system, isn't it?), episodes of foreign rule, and rulers were compared to rulers from the same country.
"The negative effects of inbreeding were not understood until the 20th century; if anything, rulers believed that inbreeding helped to preserve ‘superior’ royal traits," the team write. "In addition, the full degree of consanguinity (genetic similarity) was unknown due to complex, interrelated family trees over generations."
Unsurprisingly, they found that "European royal families did not defy the laws of biology", and that there was a "progressive decline in intelligence as inbreeding increases.” What's more, they found that ruler ability was strongly associated with levels of inbreeding, the more inbred a ruler was, the worse they performed.
Carlos II, for instance, was physically disabled and had significant mental problems, as well as being born with disfigurements – likely as a result of the inbreeding of his ancestors. He couldn't walk until the age of 4, nor talk until the age of 8.
"The resulting power struggles between factious rivals to influence Carlos II did not aid in solving the domestic and foreign challenges Spain faced."
Following his death, a new dynasty took over. Some time later, Carlos III (no relation).
"Importantly for our instrumental variable strategy, Carlos III’s parents were cousins of third degree," the team wrote, which is pretty great for royals of the time "and the hidden component of inbreeding was about that of first cousins, both substantially smaller than that of Carlos II a century earlier."
Under his rule, Spain flourished.
The team points out that the correlation between levels of inbreeding and the success of a monarch was tempered by having active parliaments that could limit the powers of the rulers. Despite surges of inbreeding among dynasties between the 15th and 18th century, they found that parliaments in Northern Europe shielded their states from the adverse effects seen elsewhere.
So even though we wouldn't recommend it for many reasons, the inbreeding of monarchies today is unlikely to have a pronounced effect on their lands – other than really, really icking them out.