Myth Check: The Habsburg Jaw?
Pop culture blames Habsburg inbreeding for this genetic defect; history, however, not so much.
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It is true that a number of prominent European monarchs of the famous Habsburg family shared an unfortunate genetic trait, mandibular prognathism, in which the lower jaw projects so far forward that some have difficulty closing their mouths and cannot make their upper and lower teeth meet. It came to be known, somewhat unkindly, as the Habsburg Jaw.
It is also true that, for a certain period of time, Habsburgs intermarried with one another. This was not done haphazardly or recklessly — the Church forbade it, so when it happened it required special dispensation — but where necessary to maintain political ties between two branches of the family. This was the period when Habsburgs occupied both the Spanish and the Austrian thrones. Nevertheless, the family intermarriage was genetically unhealthy, therefore many have speculated that this was the cause of the jaw, to the point that it’s become essentially a pop culture given. At least one scientific paper was published claiming to have confirmed this, by studying family portraits (not a super scientific method).
Those researchers may have done better to have studied the family history in more detail, though. They are correct that such intermarriage can indeed propagate (even initiate) the recessive gene for mandibular prognathism, and they are correct that there was Habsburg intermarriage. It was confined to a ~170 year period, and according to the newly published book The Habsburg Way by Eduard Habsburg, there were 73 marriages between the two family branches which included:
…four were uncle to niece, eleven marriages were between first cousins, four first cousins once removed, eight were second cousin alliances, and most of the other only one or two steps removed.
Certainly, this family intermarriage was of a sufficient degree to place blame for the Habsburg Jaw. However, that’s not the whole story; it turns out that history is not consistent with the Habsburgs’ marriage habits being the cause of the gene.
Habsburgs married outside their family before and after this period of the need to keep ties between the two branches, which was about 1526-1700. But Holy Roman Emperor Charles V (1500-1558) was born well before this period, and is famous for his Habsburg Jaw. His grandfather Maximilian I (1459-1519) had it as well — and these are only the most famous names from several generations when the jaw was found scattered throughout the family. The gene is believed to have come into the Habsburg family through Maximilian I’s grandmother Cymburgis of Masovia (1394?-1429).
The appearance of this genetic trait does not correspond temporally to Habsburg intermarriage.
In short, the extraordinarily famous Habsburg family may have popularized mandibular prognathism and become the unwilling namesakes for it; but to whatever degree its cause was intermarriage, that intermarriage was not their own.