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King Charles II was the final male heir to the Habsburg line of Spain. The product of a severe amount of inbreeding, including first cousins who married and uncles who married nieces, King Charles II's rule was marred by his ineffectuality and his severe disabilities. The king's genomes were even more homozygous than the average child whose parents are siblings. Scholars say that he was infertile, unable to chew, constantly drooled, had a tongue that was too large for his mouth, suffered from rare genetic disorders like pituitary hormone deficiency and renal tubular acidosis. Despite his likely painful life and death at the age of 38, a new theory suggests that the extremely inbred line of rulers may have evolved to eliminate the worst effects of inbreeding along the way.

According to Nature, researchers from the University of Santiago de Compostela in Spain tracked the marriages, births and deaths of 4,000 people over the course of 20 generations and three centuries. The team also took note of 502 pregnancies that led to 93 deaths of babies and 76 deaths of children.

Inbreeding is a problem because it increases a person's risk for receiving two copies of recessive mutations that lead to rare genetic disorders. Researchers hypothesized that inbreeding were also likely the culprit of early deaths. They suggested that, if natural selection was at work, fewer royal childhood deaths would occur over time.

When controlling for inbreeding, researchers found that fewer children died between 1600 and 1800, than did between 1450 and 1600. However, infant mortality increased over time. Researchers believe that mutations that often cause illness and death, like the ones that cause cystic fibrosis, are more likely to be purged early on through evolution. Meanwhile, they say that infants were more likely to die from genetic mutations that caused illness only some of the time.

However, some researchers are not convinced by the findings, suggesting that the results are caused by a statistical fluke and a small sample size. They note that the most predominant theory would suggest that most of the worst effects of sustained inbreeding would occur during the first year of life, not in older people.

The paper was published in the journal Heredity.