Royal inbreeding and the creation of the Habsburg Jaw

For nearly 500 years, the Habsburg dynasty dominated Europe, building their empire through a network of family marriages that became increasingly problematic. While they celebrated their "royal blood," they were unknowingly engaging in one of history's most extraordinary genetic experiments, with consequences that would ultimately lead to their downfall.

Visit any major European museum, and you’ll likely encounter portraits of Habsburg rulers with a striking feature: the "Habsburg Jaw." This pronounced protrusion of the lower jaw and lip, paired with a drooping nose, wasn’t just a natural variation. As science confirms, it was the result of centuries of inbreeding—an unintended consequence of their quest to preserve power within the family.

“These facial traits are remarkably consistent across generations,” says Roman Vilas from the University of Santiago de Compostela, whose 2019 study shed light on these features. “What’s truly fascinating is how they grew more pronounced as the family tree became increasingly interwoven.”

Marriage as strategy

The Habsburgs built their empire through what they called "strategic marriages," which could also be seen as a relentless pattern of marrying their closest relatives. Their motto, "Let others wage war; you, happy Austria, marry," masked the darker side of this practice—a side that would reveal itself only after centuries.

This wasn’t an occasional cousin match for convenience; the Habsburgs turned intermarriage into policy. Uncles married nieces, cousins married cousins, and the dynasty’s genealogical knots grew tighter with each generation. Over time, their unique facial features became impossible to ignore.

Vilas and his research team demonstrated that the closer the genetic relationship between Habsburg parents, the more pronounced the physical deformities in their offspring.

The Study

In their study, the researchers analyzed 66 portraits of 15 Habsburg family members. These portraits were selected based on two key criteria: the availability of high-quality images from major art museums, including the Prado Museum in Madrid and Vienna’s Museum of Art History, and historical evidence that the artists had personally observed their royal subjects. Despite differences in artistic style, these paintings provided reliable depictions of human features.

Facial surgeons used these portraits to evaluate the severity of mandibular prognathism—the jutting lower jaw—along with other deformities. They identified 11 traits related to the jaw and seven linked to upper jaw defects, including the drooping nasal tip and an extreme lower lip. Their analysis revealed that Philip IV of Spain (ruler from 1621 to 1640) had the most severe mandibular deformity, while Charles II, the last Spanish Habsburg king, displayed extreme upper jaw defects.

The researchers also mapped the family tree of the Habsburgs, which spanned 6,000 individuals across 20 generations. By comparing genetic data from the tree with their analysis of the portraits, they confirmed that inbreeding amplified recessive traits, making these facial deformities increasingly severe over time.

The genetic culmination

No Habsburg ruler illustrates this genetic crisis better than Charles II of Spain. His family tree was so convoluted that his 32 great-great-great-grandparent positions were occupied by just 10 individuals. The result was a monarch who struggled to eat, speak, and rule. Contemporary accounts described him as "weak, short, and rather like an imbecile"—a tragic end to the family’s pursuit of royal superiority.

Charles II’s inability to produce an heir marked the dynasty’s genetic dead end. His death in 1700 triggered the War of Spanish Succession, a global conflict that reshaped European power dynamics. His life symbolized the ultimate cost of the Habsburgs’ inbreeding practices.

A broader pattern

This genetic decline wasn’t unique to Charles II. Maximilian I, who initiated the Habsburgs’ marriage policy, displayed similar features despite being born to relatively distant parents, Habsburg Friedrich III and Eleanor of Portugal. His marriage to Mary of Burgundy continued the pattern, producing descendants like Charles V, whose reign was famously described as an empire "where the sun never set."

After Charles V abdicated in 1556, the Habsburg territories split into Spanish and Austrian branches, yet both continued intermarrying. Philip II, III, and IV all married Austrian princesses, intensifying the genetic issues. Philip IV’s second marriage—to Maria Anna of Austria, who was originally intended for his deceased son—produced Charles II. While Philip IV fathered healthy children with other women, his children with Maria Anna were plagued by deformities.

The Austrian Habsburgs fared little better. Emperor Ferdinand III married his cousin Maria Anna (daughter of Philip III) and later married another close relative, Maria Leopoldine of Austria-Tyrol. By the time Charles VI—the last male Austrian Habsburg—died in 1740, the dynasty’s genetic challenges had come full circle. His daughter Maria Theresa secured the throne but only after waging a brutal war of succession, keeping the Habsburg legacy alive for another 178 years.

The modern-day Habsburg descendants show significantly less pronounced features of the Habsburg jaw than their historical relatives. This is because after the end of Habsburg rule, the family began marrying outside their close family circle, introducing new genetic diversity. The current head of the House of Habsburg-Lorraine, Karl von Habsburg, and his relatives show some mild facial features that might be traced to their Habsburg ancestry, but nothing as severe as what was seen in historical figures like Charles II of Spain or his relatives. This improvement in genetic health occurred because the family abandoned the practice of marriages between close relatives that had characterized their dynasty for centuries. Some modern Habsburgs may have a slightly prominent lower jaw, but it's typically within normal variation and not the extreme feature that became legendary in their ancestors' portraits.

The dynasty that once ruled much of Europe learned that while they could arrange marriages and control empires, they couldn’t escape the rules of biology. Their obsession with consolidating power through family alliances created a genetic bottleneck, and their once-mighty empire collapsed.