The exorcism took place in Madrid, in the final years of the seventeenth century, inside a palace where the king of an empire that stretched across oceans lay wasting away. The royal confessor, a Dominican friar named Fray Froilán Díaz, was convinced that the king’s affliction was not natural. Under the supervision of the Grand Inquisitor, Juan Tomás de Rocabertí, Díaz worked to break a spell he believed was destroying the monarch from within. Three centuries later, geneticists rebuilt that same king’s family tree.

It ran sixteen generations deep and contained more than three thousand people. When they calculated his inbreeding coefficient, the number came back higher than what you would expect for a child born of a brother and sister. His grandfather was also his great-grandfather. This was Charles II, the last Habsburg king of Spain, and one of the most studied cases of royal inbreeding in history.

The number looks like a puzzle. It is not. In 2009, a team at the University of Santiago de Compostela calculated the inbreeding coefficient for every Spanish Habsburg monarch. The simplest way to picture it is this: you carry two copies of almost every gene, one from your mother and one from your father.

Imagine flipping a coin at every point in your DNA and asking, “Are both of these copies secretly the same version, inherited from a common ancestor through two different paths? ” The inbreeding coefficient is the probability that the answer is yes. For Charles II, that probability reached 0. 254, roughly one in four.

One in four is nearly what you would expect for a child whose parents were a brother and sister, or a parent and their own child. Charles’s number came out slightly higher than that. None of the Spanish Habsburg kings ever married his sister, not once. They reached that figure without ever crossing that line.

Be careful about what the number represents. No one has ever exhumed Charles’s remains and tested his DNA. This was mathematics performed on a family tree, a probability calculated from who married whom across more than three thousand people. It is powerful.

But it is also just paper. Keep that in mind, because it will matter later. What stands out from the numbers is the sharp ascent. Philip I, the founder of the Spanish dynasty, sat at 0.

025. Five generations later, Charles stood at ten times that. Tenfold in five steps. So what happened in between?

Marriages. Deliberate marriages. Charles I of Spain, the man the rest of Europe called Emperor Charles V, married his first cousin, Isabella of Portugal. Their son, Philip II, went further.

His first wife, Maria of Portugal, was his cousin on both sides, meaning they shared not one set of grandparents but both sets. Later, Philip II married his own niece, Anna of Austria. Then came Philip IV. Philip IV is the one that is hard to stop thinking about.

His heir, Baltasar Carlos, was supposed to marry an Austrian archduchess named Mariana. Mariana was Philip IV’s own niece. Then, in 1646, Baltasar Carlos died at the age of sixteen. The dynasty had a bride without a groom.

So the king married her himself. His son’s intended bride. His niece. His sister’s daughter.

That marriage is the source of the puzzle that started all of this. Mariana’s mother was a daughter of Philip III. Philip IV was also a son of Philip III. So when Mariana and Philip IV had a son, Philip III appeared in that boy’s family tree twice, once as his grandfather on his father’s side, and once as his great-grandfather on his mother’s side.

Two branches of the tree that should have diverged folded around each other instead. This weakens the easy explanation. It was not simply that there was no one else to marry. These were rulers connected to royal houses across Europe.

Their marriages were political instruments, used to strengthen alliances, protect inheritance claims, and keep power within the wider Habsburg family network. The motives varied from marriage to marriage, but the repeated preference for relatives, generation after generation, made close kinship a built-in feature of the dynasty rather than an accident. And for a long time, the strategy worked. That raises an obvious question.

If something was happening to their bloodline, would you be able to see it? Probably. You have likely already seen it on the walls of museums. The Habsburg jaw, that heavy protruding lower jaw, the thick lower lip, the face that seems pushed forward slightly from below.

It appears in painting after painting over two centuries. For a long time, the standard explanation was simple: it was just a family trait, perhaps passed down by a single dominant gene, the way some families share the same nose. In 2019, a team led by Román Vilas tested that. They gave ten maxillofacial surgeons, people whose profession is repairing jaws, sixty-six paintings of fifteen members of the dynasty.

The surgeons evaluated eleven traits of lower-jaw protrusion and seven traits of midface deficiency: sunken midface, drooping nasal tip. The most extreme lower jaw belonged to Philip IV, the man who married his niece. Among those with the most severe midface deficiency was Charles II. The researchers then compared those results with the inbreeding numbers.

The more inbred the Habsburg, the more pronounced the lower-jaw protrusion. Inbreeding accounted for about 22 percent of the variation in that jaw’s severity from one family member to another. The researchers interpreted this as a sign that the trait had a recessive component, something hidden that emerges more strongly when the same version arrives from both sides. Now, pause.

These were paintings. Velázquez was a staggeringly realistic painter, but a portrait is not an X-ray, and fifteen people is a very small sample. The researchers themselves said they could not conclude that the jaw was definitely caused by inbreeding. The 22 percent also means that most of the variation came from something else.

But think about what it suggests. The families’ own court painters, hired to make them look magnificent, recorded the pattern anyway, painting after painting. And these were only the ones who lived long enough to sit for a portrait. What happened to the children who did not?

Here is the number that changes the whole picture. Between 1527 and 1661, the Spanish Habsburg royal families produced thirty-four children. Ten of them, 29 percent, died before their first birthday. Seventeen, exactly half, died before the age of ten.

In Spanish villages during the same era, the infant mortality rate was about 20 percent. Think about that. Royal physicians, palace care, every advantage money could buy, and still Habsburg children died at unusually high rates. Researchers found something finer than that average.

The higher a child’s inbreeding coefficient, the lower their chance of reaching ten years old. That relationship was statistically significant, which simply means it is unlikely to be a coincidence. It is a pattern across the whole family, not a verdict on any single child. Children died for countless reasons in the seventeenth century.

You cannot point at a small coffin and say, “This one was because of the bloodline. ” But you can watch how the pattern looked inside a single marriage. Philip IV and Mariana, uncle and niece, had their first child in 1651. Margarita Teresa survived.

She is the little girl at the center of Velázquez’s most famous painting, “Las Meninas. ” Then came a daughter, María Ambrosia, in 1655. She was described as suffering from epilepsy and lived only fifteen days. Two years later came a son, the boy the astrologers promised would live.

Keep him in mind. Then another son, Thomas Charles, born in 1658, died as an infant. And then, in November 1661, Charles was born. Four years later, his father died, and Charles became king of Spain at the age of three.

His mother Mariana ruled as regent for the next ten years. So the child at the top of the inbreeding chart, the one with the highest coefficient in the calculations for the Spanish Habsburg kings, was now responsible for Spain, its empire in the Americas, its territories in Italy and the Low Countries, all of it. The 2009 paper did not soften what came next. It describes Charles plainly as physically and intellectually disabled.

He married twice. Neither marriage produced a single child. So what exactly was wrong with him? Here, the geneticists made their boldest move.

Gonzalo Álvarez and his colleagues proposed that Charles may have had not one genetic disorder but two, entirely unrelated to each other. The first was combined pituitary hormone deficiency. The pituitary is a small gland at the base of the brain that acts as a control panel for growth, development, and fertility. The second was distal renal tubular acidosis, where the kidneys cannot properly clear acid from the blood, which can stunt growth and weaken bones.

The specific forms of both disorders that the researchers proposed are recessive. To be affected by either one, an individual needs a defective copy from both parents, and the two disorders sit on separate, unlinked genes. Remember the coin flip from the beginning. Charles would have needed that unlucky result in two different places at once.

For most people, that is extremely unlikely. But for a family tree that keeps folding over itself, it is far less unlikely. The researchers speculated that this combination could explain much of what was recorded about his health, including his apparent inability to father children. If they were right, then the marriages meant to protect the dynasty’s inheritance had helped produce the body that could not pass it on.

Notice the word the authors themselves used: “speculate. ” It is a hypothesis, not a diagnosis. No one has tested Charles’s DNA. Other researchers, working from the same old descriptions of the king, have proposed completely different conditions, including aspartylglucosaminuria, Klinefelter syndrome, and fragile X syndrome.

Each one is a modern guess, built on records written by people who had no idea what a gene was. And some of those people believed something far stranger was happening to their king. By the late 1690s, Charles was in his thirties, still childless, and visibly ill. The Spanish monarchy still stretched across Europe and the Americas, but it had no heir.

And inside the palace, a terrifying explanation took hold. The king had been bewitched. History still calls him “Charles the Bewitched. ” This is not later gossip.

The records survive. The royal confessor, Fray Froilán Díaz, became convinced that the king’s illness was caused by a spell. In 1698 and 1699, under the direction of the Grand Inquisitor Juan Tomás de Rocabertí, Díaz performed a series of exorcisms on the king. He also held sessions with people believed to be possessed by demons, trying to find out who had cast the spell and how.

The accusations reached people at court, even the queen herself, Mariana of Neuburg, Charles’s second wife. Think about what that means. A king who cannot produce an heir, and instead of examining his body, the investigations turn toward the woman who is supposed to give him one. When the queen found out, the magic turned on Díaz.

He was removed from his post as confessor and arrested by the Inquisition on charges of heresy. His case dragged on through a long, secret legal battle. Accounts of his trial survive in archives in America and Britain. One of them describes Díaz as caught in the middle of a power struggle for control of the Spanish throne.

Inside that palace, the curse became a weapon, aimed at whoever could be blamed for the empty cradle. The legend did not stop at Charles’s death. You may have seen the famous autopsy story: no blood in his body, a heart the size of a peppercorn, a head full of water. Versions of this description appear in modern medical and popular writing, but it is difficult to trace the wording with certainty to a contemporary autopsy report.

Treat it as an unverified account, not direct medical evidence from the year 1700. Spain blamed demons. Modern writers like to blame genes. But there was another royal dynasty where brothers married sisters, generation after generation, and their story points somewhere no one expects.

Ptolemy II married his full sister, Arsinoe II. Not a cousin, not a niece, but his sister. Rather than hiding it, they advertised it. They were known as “Philadelphus,” which roughly means “sibling-loving.

” Their dynasty, the Greek-speaking family that ruled Egypt after Alexander the Great, kept doing it. Ptolemy IV married his full sister, Arsinoe III. Ptolemy VIII married his sister, then also married her daughter, who was his niece and stepdaughter, Cleopatra III. The most famous member of the entire family, Cleopatra VII, married first one of her younger brothers, Ptolemy XIII, then another, Ptolemy XIV.

So if the question is simply who married their closest relatives, the Habsburgs lose easily. Some modern genealogical reconstructions suggest Cleopatra’s theoretical inbreeding coefficient was very high, possibly around 0. 47, almost double Charles’s. But that is not a peer-reviewed estimate, and it is not the same kind of calculation as the Spanish study.

It rests on disputed assumptions about several generations of her family tree, including the identity of women whose ancestry is not precisely known. Some ancient sources even described her father as illegitimate. Change one of those links, and the number changes. Still, even on the most careful reading, the Ptolemies married their siblings again and again for generations.

So where is their collapse? Where is their Charles? The historian Sheila Ager raises an uncomfortable point. Direct inbreeding increases the risk of harm.

It does not guarantee it, and there is simply not enough evidence to prove that the Ptolemies were harmed by it. The dynasty’s most violent and infamous king, Ptolemy VIII, was not himself born of a direct incestuous marriage. His parents’ marriage cannot be blamed for his cruelty. But notice what is missing on the Egyptian side.

There is no merciless court painter like Velázquez. There are no records of royal children who died in their first year. Silence is not proof of health. Perhaps the Ptolemies suffered and left us nothing to measure.

That is what makes the Spanish case so valuable and so painful. The Habsburgs are not necessarily the most inbred family in history. They are one of the clearest royal cases where researchers can compare a deep pedigree with survival data and physical traits. And the clearest place to feel what that data means is not Charles.

It is the brother who came before him. Go back to 1657. Philip had already lost his adult heir, Baltasar Carlos. His marriage to Mariana had produced daughters, but no son.

Then the astrologers assured him the next child would be a boy, and that this boy would live. In November 1657, he was born. They named him Philip Prospero, “Philip the Prosperous. ” The name itself was a prayer.

Before he had even reached his second birthday, Velázquez painted him. That painting still exists. It hangs today in the Kunsthistorisches Museum in Vienna, and it was not painted to hang on a nursery wall. In 1659, it was sent to the court in Vienna as a gift.

This was the heir being shown to Europe. The future of Spain, standing right there. Look at him. A small boy in a baby dress, one arm resting on a chair, a small wide-eyed dog staring straight at you, and hanging from his clothes a cluster of amulets, charms for protection.

Now, be fair about the amulets. Royal children wore them often. They are not a medical record. But it is recorded that this child was delicate and sickly, and that he was ill for a long time before the end.

Philip Prospero died on November 1, 1661. He was three years old. Five days later, his mother gave birth to Charles. The infant who would become the most inbred king in history was not born amid celebrations.

He was born in a palace that had just lost its heir. A medical journal from the University of Vienna describes the first twelve years of Mariana’s marriage as a long series of pregnancies, miscarriages, and sick children. We do not know why each of them died, and no one should claim to. But the boy the astrologers promised would live did not.

The boy no one was counting on lived. He lived thirty-eight years. And in the final weeks of his life, Charles had to decide who would inherit everything his family had spent two centuries preserving. He chose Philip, Duke of Anjou, a French prince, the grandson of Louis XIV, Spain’s great rival, and also the grandson of Maria Theresa, Charles’s own half-sister.

Charles died on November 1, 1700, the same date his brother Philip Prospero had died thirty-nine years earlier. That is a coincidence, not a curse. Then came the bill. Within months, Europe was sliding toward war.

Charles’s will and the disputed succession were the spark, but behind them lay a larger fear: what would a French prince on the Spanish throne do to the balance of power in Europe. The War of the Spanish Succession began in 1701. The main peace treaties came in 1713, and the broader fighting ended in 1714. When it was over, a Bourbon sat on the throne of Spain.

His descendants still sit there today. Generations of marriages between cousins, uncles and nieces, a bride passed from son to father, all within a single family network. And in the end, the crown went to the French. In Vienna, that painting still hangs.

A little prince, a wide-eyed dog, a dress covered in amulets, sent across Europe to prove that Spain had a future. Now you know what the family hoped those amulets would prevent, and how powerless anything was to stop it. Royal sibling marriage did not begin with the Ptolemies. The pharaohs practiced it centuries before them.

A major DNA study of royal mummies concluded that the parents of one famous pharaoh were brother and sister.