When King Charles II of Spain died in 1700 at the age of 38, his body was so ravaged by illness that his autopsy became the stuff of legend. Court chroniclers later claimed his heart was the size of a peppercorn, his lungs were corroded, and his veins contained no blood. The dramatic details were likely embellishments written years after the fact, and the original report has been lost. But the clinical truth was remarkable enough on its own.
Charles II was the last Habsburg ruler of Spain, and he was the product of one of the most extreme cases of royal inbreeding ever documented. A peer-reviewed genetics paper calculated his inbreeding coefficient at 25. 4 percent, a figure higher than the offspring of a brother and sister. He was known as El Hechizado, or “the bewitched,” because contemporaries assumed sorcery was responsible for his condition.
Genetics offered a simpler explanation. The Habsburg family’s motto was attributed to the Holy Roman Emperor Maximilian I: “Let others wage war; you, happy Austria, marry. ” Around 1500, Maximilian discovered that marriage was the cheapest way to acquire territory. He married Mary of Burgundy and gained control of most of the Low Countries.
His son Philip married Joanna of Castile and inherited Spain. The strategy worked so well that the Habsburgs eventually ran out of unrelated royalty to marry. Between 1516 and 1700, 16 out of 18 marriages in the Spanish Habsburg branch were between blood relatives, nearly 90 percent. The preferred pairing was uncle and niece.
King Philip II of Spain married his niece Anna of Austria. His son Philip III married his cousin, and that son’s successor continued the pattern. Charles II was the product of one of those uncle-niece unions, and by the time he was born on November 6, 1661, generations of recycled DNA had compressed his bloodline to an extraordinary degree. Charles was the son of King Philip IV and Queen Mariana of Austria, who was Philip’s niece.
His health was a catalog of conditions that 17th-century doctors had no language to describe. His lower jaw jutted out so far that his teeth could not meet, a condition known as mandibular prognathism, now called the Habsburg jaw. His tongue was reportedly too large to control. He drooled, spoke with difficulty, and could not chew solid food.
He was not weaned until he was five years old and did not walk until he was eight. He was also infertile. His two wives, Marie Louise of Orleans and Maria Anna of Neuburg, were repeatedly blamed for their empty cradle. They were not the problem.
Researchers suspect Charles had at least two distinct recessive genetic conditions, possibly combined pituitary hormone deficiency and distal renal tubular acidosis. The honest answer, scholars say, is that the list of possibilities is long. The mechanics are well understood. The inbreeding coefficient, written as F, measures the probability that two copies of a gene in one person are identical because they came from the same recent ancestor.
Children of first cousins have an F of about 6 percent. Children of an uncle and niece reach 12. 5 percent. Children of full siblings hit 25 percent.
Charles II was at 25. 4 percent, statistically more inbred than a sibling child because the same genes had been arriving from every direction for generations. Most harmful genetic mutations are recessive. A person can carry one broken copy and never know because the working copy from the other parent does the job.
When two unrelated people have children, the odds that both carry a broken copy of the same gene are low. When relatives have children, those odds rise dramatically. The result is what geneticists call inbreeding depression: higher rates of stillbirth, infant mortality, infertility, skeletal disorders, and rare diseases that normally appear in only one in 100,000 people. The Habsburgs did not invent this practice.
The Ptolemies, a Greek family that ruled Egypt for nearly three centuries, married siblings for almost their entire dynasty. They justified it religiously by pointing to the divine sibling couple Isis and Osiris and politically by keeping land and crowns inside the family. The result was 11 generations of sibling marriage. Cleopatra VII, the famous last queen of Egypt, was the daughter of a brother-sister marriage.
She then married her younger brother Ptolemy XIII when she was about 18 and he was about 10. After he drowned during their civil war, she married her even younger brother Ptolemy XIV, who was about 12. He died soon after, probably poisoned. By the time she allied with Mark Antony, Cleopatra had been a wife of two of her own brothers and the product of generations of genetic recycling.
The 18th dynasty of Egypt arrived at the practice even earlier and perhaps more severely. In 2010, a study led by Egyptologist Zahi Hawass published in the Journal of the American Medical Association concluded that King Tutankhamun’s parents were full siblings. His mother was a mummy labeled KV35 Younger Lady, and she was the biological sister of his father, Akhenaten. Tut’s inbreeding coefficient was around 15.
5 percent. CT scans of Tut’s mummy revealed a cleft palate and a clubfoot with signs of Kohler disease, a rare condition in which bones die from poor blood supply. He had likely contracted malaria more than once. In his tomb, archaeologists found 130 walking sticks, some bearing signs of actual use.
The golden-masked boy king had spent his real life leaning on a cane. He died at about age 18 or 19, likely after a broken leg became infected. Queen Victoria of the United Kingdom offers a different kind of royal genetic story. She ruled for 63 years, and her nine children married into royal families across Germany, Russia, Spain, Sweden, Norway, Greece, and Romania.
She carried a new mutation in the F9 gene on the X chromosome, causing hemophilia B, a deficiency in clotting factor nine. Her parents showed no signs, so researchers believe the mutation was spontaneous, possibly in her father’s reproductive cells. Through her carrier daughters, the mutation spread into the Spanish, German, and Russian royal families. In Russia, her granddaughter Alexandra married Tsar Nicholas II, and their son Alexei inherited hemophilia B.
The boy’s suffering led the court to accept the influence of the mystic Rasputin, whose role in the fall of the Russian monarchy has been debated for a century. In Spain, Victoria’s great-grandson Alfonso XIII had two sons with hemophilia B. Both died after relatively minor car accidents in the 1930s. Victoria and her husband Albert were first cousins, a common practice in 19th-century Europe.
That marriage did not cause the hemophilia. The mutation was new and would have existed regardless of who Victoria married. Europe was not the only continent practicing royal incest. In pre-contact Hawaii, the highest-ranked chiefs married siblings to concentrate mana, or sacred spiritual power.
In the Andes, Inca emperors were required to take their full sister as principal queen, following the example of the sibling gods Inti and Mama Quilla. When the Inca successors Huascar and Atahualpa fought over the throne, their empire was already facing the arrival of Francisco Pizarro, guns, and smallpox. In Persia, the Sasanian Empire practiced xwedodah, next-of-kin marriage, which Zoroastrian texts treated as virtuous. After the Islamic conquest in the 7th century, the practice was banned hard and essentially vanished.
In 19th-century Siam, now Thailand, King Chulalongkorn married several of his half-sisters as standard royal practice. An old law forbade commoners from touching any royal body. In 1880, his queen Sunanda fell from a royal boat into a canal, and eyewitnesses reportedly stood frozen, because saving her would have required breaking the law. She drowned.
The law was reformed soon after. Researchers have tried to quantify what these dynasties did to themselves. In 2019, a team led by Roman Vilas at the University of Santiago de Compostela published a study in Annals of Human Biology using facial morphometric analysis on portraits of Spanish Habsburg kings and queens. They measured the jutting jaw, the receding upper lip, and the gap between the front teeth, then correlated those measurements with each subject’s documented inbreeding coefficient.
The correlation was 0. 89, nearly a perfect line. Charles II died on November 1, 1700, two months after his 39th birthday. The autopsy was held the next day.
The official record was short and clinical. The dramatic versions came later from courtiers with a literary streak. The Spanish throne passed to a French Bourbon teenager, and 12 European countries spent the next 13 years fighting the War of Spanish Succession over the decision. Estimates of the dead range from 700,000 to 1.
2 million. The Austrian branch of the Habsburgs survived, married widely, and has descendants alive today with ordinary faces. The Spanish branch ended on a hospital bed in Madrid in 1700. The number associated with Charles II remains the strangest thing royalty ever measured: 25.
4 percent.
