How Centuries of Inbreeding Led to the Distinctive 'Habsburg Jaw'

The Habsburgs (spelling it Hapsburg is an Americanization) were one of the most successful ruling families in all of Europe, wielding power in a big way from the 13th century to 1918. At one time, the monarchy's vast empire included Austria, Hungary, Bohemia, Slovenia, Slovakia and Croatia, as well as large parts of Poland, Romania and Italy.

The royal family really began its ascent to power in 1273, when Rudolf I became the Roman German king. Ottokar II Premysl, who was the king of Bohemia, refused to recognize Rudolf, which sparked a royal rivalry. Ottokar became king of Austria, but when he was killed in battle, Rudolf moved in, and granted land to his sons. And that's how the Habsburgs got their start in Austria.

The Habsburgs spent the next hundred years or so empire building. They took over the Tyrol, a region in the Alps shared by Austria and Northern Italy, in 1363. Things really took off in the 1500s, when future emperor Maximilian married Charles the Bold's daughter Mary and gained control of the desirable Burgundy region. This elevated the family to A-list royal status.

The Habsburgs came into their full regal power in the 1600s and enjoyed its fruits through the early 1700s. The Habsburg royal clan included Leopold Wilhelm, an emperor, bishop and patron of the arts; reformer Joseph I whose motto was "by love and fear"; Charles VI who ruled Spain and set up his daughter to take the crown; and Rudolf II, who decided to live in Prague, where he promoted science and art. It was during this era that royal watchers noticed a striking similarity in the jawline of several members of the Habsburg family, and dubbed it the Habsburg jaw.

Habsburgs including Joseph I, Charles I of Spain, Leopold Wilhelm and Charles II were known for having a very prominent lower jaw. According to a 1988 article in the Journal of Medical Genetics, nine successive generations of the Habsburg family had this pronounced jawline, so it started to be commonly known as the Habsburg jaw.

The medical term for this kind of jaw is mandibular prognathism, which is when the jaw juts so far forward that it causes an extreme underbite. It doesn't just change the shape of the face; the teeth don't line up as they should. Researchers believe the Habsburgs also had a mandibular deficiency, which is a pattern of abnormalities, including skeletal, neuromuscular, occlusal and esthetic conditions that can affect a person's speech and their ability to eat.

In his book "Spain Under Charles the Second", Alexander Stanhope, an 18th-century British envoy, wrote of 35-year-old Charles II:

The trait showed up more often in Habsburg men, though it affected female Habsburgs too, including Mariana of Austria, who was the queen of Spain. Spanish painter José García Hidalgo immortalized royal sisters Maria Luisa de Orléans, queen of Spain, and sister Anne Marie de Orléans, queen of Sardinia. The portraits are attractive, but the Habsburg family jaw is unmistakable.

Researchers now know that the Habsburg jaw was a result of inbreeding, which is when close relatives have children together. A team led by genetics researcher Román Vilas from the University of Santiago de Compostela in Spain confirmed this in late 2019. In the study, which was published in the journal Annals of Human Biology, they analyzed portraits from 66 members of the Habsburg clan, zeroing in on 15 monarchs who had the family's pronounced jawline and underbite. The team found 18 different abnormalities in the Habsburgs' faces, and used the portraits to calculate the degree to which different members of the Habsburg family were inbred.

The researchers found that Habsburgs had both mandibular prognathism and mandibular deficiency. The effects of intermarriage caused deformities that followed a recessive pattern, and showed up in the lower third of the face.

Most societies have long held taboos against incest and inbreeding. These prohibitions often are morally based, but we now know there are vital scientific reasons why inbreeding between first cousins or siblings is a bad idea, says Montgomery Slatkin, a retired genetics professor from University of California, Berkeley.

Each of us has two copies of each chromosome. In order for a recessive trait, like blue eyes or Type O blood to show up, a person needs homozygous alleles. That means you need two sets of chromosomes that both carry the same recessive trait.

"Any condition that's recessive, meaning you need both copies of the allele to have the physical defect, are much more common among inbred children than it is among people who are not inbred," Slatkin says. "It's hundreds of thousands times more common."

When you have children with someone who's not closely related to you, usually most of your genes will be heterozygous, which means they carry different traits. When cousins marry, it raises the odds of similar genetic background — and birth defects.

Royals in the Habsburg era tended to ignore the proscriptions against intermarriage and incest, because they were kings and queens after all. They wanted to consolidate power and property, keeping it in the family. Unfortunately that intensified familial genetic defects and flaws too, to the point that Charles II suffered with many maladies.