PLoS One, scientists from Spain’s University of Santiago de Compostela argue that inbreeding so incapacitated the Habsburgs over the centuries that by the death of King Charles II of Spain in 1700, they were virtually unable to reproduce. From 1516 to 1700, it has been estimated that over 80% of marriages within the Spanish branch of the Habsburg dynasty were consanguineous; that is, they were marriages between close blood relatives. Most often, these unions took the form of marriages between first cousins, double-first cousins, and uncles/nieces. Conceivably as a direct result of these marriages between relatives, infant and child mortality rose to 50% among Spanish Habsburgs, much higher than the average for the period. But the final Habspurg king of Spain, Charles II, was perhaps the most unfortunate result of these unions. Also know as “El Hechizado” (“The Hexed”), Charles was severely deformed. The so-called “Habsburg Lip”, a form of mandibular prognathism often seen among members of the Habsburg Dynasty, was so pronounced in Charles’ case that it was difficult for him to speak. An enlarged tongue, gastrointestinal problems, mental retardation, and possible growth problems meant that Charles was raised almost as an infant until the age of 10. Even as he grew older, he was never able to govern effectively. His rule saw the rapid decline of the Empire, only exacerbated by his death in 1700. But for all the speculation and anecdotal evidence of the negative impact of inbreeding on the House of Habsburg, there has been little scientific research as to whether inbreeding actually played a factor in its extinction. The authors of this study sought to achieve this goal by examining genealogical information, in the form of family pedigrees, for the eight royal families connected with the Habsburg dynasty. All told they analyzed family pedigrees of over 3,000 individuals spanning 16 generations. They then used this information to calculate the inbreeding coefficient for each family member. The inbreeding coefficient is simply a measure of the chance that someone will receive an identical set of genes from both parents. Unsurprisingly, the authors found elevated inbreeding coefficients that for many Habsburgs. In fact, the levels increase consistently from the earliest Spanish Habsburgs, like King Philip I (1478-1506), to Charles II, the last Spanish Habsburg king. Even more interestingly, some of the Habsburgs – notably Charles II – had an inbreeding coefficient nearly twice what one would expect given the level of relatedness between his mother and father. In other words, even though Charles’ parents were related to each other as uncle and niece, his inbreeding coefficient fell at the same level as someone whose parents were brother and sister. These unexpectedly high levels indicate that consanguineous marriages, like that of Charles’ parents, had probably been happening along the Habsburg line for hundreds of years. This practice came to a head with the birth of Charles, whose inbreeding coefficient was the highest of all the Spanish Habsburgs, and whose physical deformities were the most severe. Further, the authors argue that Charles’ ill health was a direct result of centuries of consanguineous unions. Specifically, they point to a growth hormone deficiency and severe renal tubular acidosis, which may have accounted for his short stature and his many physical deformities and ailments. While these diseases are quite rare in the general population, the fact that so many of Charles’ ancestors were related to each other would have increased his chances of inheriting the genes associated with them. Whether Charles did in fact suffer from these specific diseases is still open to interpretation, though it is clear that his physical and mental difficulties prevented him from fathering any heirs to the throne. The Habsburg dynasty in Spain ended when Charles passed away in 1700, a few days shy of his 39th birthday.