The family tree of the Habsburgs, a German-Austrian ruling family whose domain stretched from Portugal to Transylvania, is a tangled one. Like many royal families, the Habsburgs made strategic marriages to consolidate their power, often to close relatives.
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And while the dynasty’s regalia was glittery and their palaces splendid, the royals themselves were markedly less easy on the eyes: Generation after generation, Habsburg monarchs had sharply jutting jaws, bulbous lower lips and long noses. This distinctive “Habsburg jaw,” a new analysis published in the Annals of Human Biology finds, most likely resulted from inbreeding.
The researchers, led by geneticist Román Vilas from Spain’s University of Santiago de Compostela, focused on 15 members of the so-called Spanish Habsburgs. While the Habsburg family rose to power in central Europe as the rulers of Austria, Germany and eventually the Holy Roman Empire, the family's influence spread westward to Spain after Philip I, son of the second Habsburg Holy Roman Emperor, married Joan of Castile in 1496 . The Spanish Habsburgs' reign lasted two centuries, until the 38-year-old Charles II, a king whose manifold health woes and infertility scholars often attribute to severe inbreeding, died in 1700 with no immediate heir.
Vilas and his colleagues honored in on Spanish Habsburgs whose appearances artists—including notables like Diego Velázquez—had documented in photorealistic portraits. Using an extensive family tree spanning 20-plus generations, the scientists determined that the average inbreeding coefficient of the Habsburgs they analyzed was .093. This means that roughly 9 percent of a given royal’s corresponding genes (one maternal, one paternal) were identical because they came from the same ancestor, according to Ed Yong of National Geographic. (Comparatively, the child of two first cousins would have an inbreeding coefficient of .0625, and the child of two third cousins, like England’s Prince Charles, would have an inbreeding coefficient of .004.)
In addition to quantifying how inbred each aristocrat was, the researchers asked mouth and jaw surgeons to look at the portraits and determine how many abnormal facial features typical of mandibular prognathism (MP, or protruding jaw) and maxillary deficiency (sunken midface) each Habsburg possessed . Higher scores indicated stronger occurrence of dysmorphic features.
Vilas’ team found that unfortunate-looking Habsburgs with high MP scores—that signature “Habsburg jaw”—were more likely to have a high inbreeding coefficient. In fact, differences in levels of inbreeding accounted for 22 percent of the differing severity of mandibular prognathism among the Habsburgs studied.