A facial condition called the ‘Habsburg jaw’ (mandibular prognathism) owes its name to its high prevalence in the Habsburg dynasty of Spanish and Austrian kings and their wives. The members of this dynasty are characterized by other signs of facial deformity, including an everted lower lip, also known as the ‘Habsburg lip,’ and noses with a hump and overhanging nasal tip, also known as the ‘Habsburg nose,’ which are often indicative of maxillary deficiency. According to new research, published in the Annals of Human Biology, the Habsburgs’ distinct chin was a result of the high prevalence of consanguineous marriages in the dynasty.
King Charles II of Spain was the last in the Habsburg line and one of the most afflicted with the facial deformity.
“The Habsburg dynasty was one of the most influential in Europe, but became renowned for inbreeding, which was its eventual downfall,” said lead author Professor Roman Vilas, a researcher in the Department of Zoology, Genetics and Physical Anthropology at the University of Santiago de Compostela.
“We show for the first time that there is a clear positive relationship between inbreeding and appearance of the Habsburg jaw.”
Professor Vilas and colleagues recruited 10 maxillofacial surgeons to diagnose facial deformity in 66 portraits of 15 members of the Habsburg dynasty. Despite differences in artistic style, the portraits are characterized by a realistic approach to the human face.
The surgeons were asked to diagnose 11 features of mandibular prognathism and 7 features of maxillary deficiency.
The surgeons gave scores for the degree of these conditions in each member of the Habsburg family.
Mary of Burgundy, who married into the family in 1477, showed the least degree of both traits.
Mandibular prognathism was most pronounced in Philip IV, King of Spain and Portugal from 1621 to 1640.
Maxillary deficiency was diagnosed to the greatest degree in five members of the family: Maximilian I (regent from 1493), his daughter Margaret of Austria, his nephew Charles I of Spain, Charles’ great-grandson Philip IV and the last in the Habsburg line, Charles II.
The scientists detected a correlation between the two conditions, suggesting that ‘Habsburg jaw’ is in fact characterized by them both and that they share a common genetic basis.
The extent of inbreeding was calculated from a large-scale family tree, including more than 6,000 individuals belonging to more than 20 generations.
The researchers detected a strong relationship between the degree of inbreeding and the degree of mandibular prognathism.
The relationship to maxillary deficiency was also positive, but it was only statistically significant in two of the seven features diagnosed.
“The causes of the relationship between inbreeding and facial deformity remain unclear, but it’s because the main effect of mating between relatives is an increase in the chances of offspring inheriting identical forms of a gene from both parents, known as genetic homozygosity,” Professor Vilas and co-authors said.
“This reduces people’s genetic fitness, so Habsburg jaw should be considered a recessive condition.”
“While our study is based on historical figures, inbreeding is still common in some geographical regions and among some religious and ethnic groups, so it’s important today to investigate the effects,” Professor Vilas added.
“The Habsburg dynasty serves as a kind of human laboratory for researchers to do so, because the range of inbreeding is so high.”
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Román Vilas et al. Is the ‘Habsburg jaw’ related to inbreeding? Annals of Human Biology, published online December 2, 2019; doi: 10.1080/03014460.2019.1687752
