For centuries, royal families across Europe used marriage as a tool of statecraft, binding alliances and concentrating power within a single dynasty. Some went further, marrying close relatives generation after generation to keep wealth and titles in the family. The result was a pattern of congenital illness, infertility, and dynastic decline that historians still study today.
Inbreeding occurs when closely related people have children. It raises the odds that a child will inherit two copies of the same recessive gene, which can produce genetic disorders. Scientists measure this with the inbreeding coefficient, a scale running from zero, meaning no inbreeding, to one, meaning completely inbred.
When a person's parents are first cousins, the inbreeding coefficient is 0.0625, a 6.25 percent chance of inheriting the same gene from both sides. For siblings or the offspring of a parent and child, the coefficient is 0.25, a 25 percent chance. The higher the number, the greater the risk that recessive conditions will appear.
The most infamous example of royal inbreeding is the House of Habsburg. Once one of Europe's most powerful families, the Habsburgs held the throne of the Holy Roman Empire from 1438 until 1740. By the 16th century, they controlled lands in Spain, the Netherlands, France, Italy, Austria, and the Balkans.
To preserve political power and control over their territories, the Habsburgs routinely married within the family. Uncles married nieces, and first cousins married each other. Their family tree became so tangled that the term tree hardly applies, and the genetic similarities grew more pronounced with each generation.
The most visible consequence was a condition known as the Habsburg jaw, a pronounced and protruding lower jaw. It was not merely cosmetic. The deformity made it difficult for sufferers to chew or speak properly, causing further complications and pain.
King Charles II of Spain is perhaps the most notable member of the dynasty. His inbreeding coefficient was 0.25, even though his parents were not brother and sister. They were uncle and niece, and both were themselves heavily inbred.
Charles was severely disabled. He had the Habsburg jaw and a large tongue, and it was said he could not speak until he was four years old and swallowed his food whole. As a child he contracted measles, rubella, and smallpox, and it has been speculated that he also suffered from hormone deficiency and kidney failure.
He struggled to walk and to write, and he suffered from depression. His many conditions and illnesses earned him the nickname Charles the Hex. He was likely impotent and never produced an heir, which led to conflict over his lands after his death.
His autopsy described him in harsh terms, noting a heart the size of a peppercorn, corroded lungs, rotten and gangrenous intestines, a single testicle as black as coal, and a head full of water. Yet despite his detractors, he was noted for kindness toward foreign dignitaries and a love of the outdoors.
Charles was the worst affected by his family's inbreeding, but he was far from alone. Don Carlos of Spain, eldest son of King Philip II and his second wife, Queen Maria of Portugal, is another tragic case. Born in 1545, he was the child of cousins who were themselves the product of inbreeding.
Carlos had only four great-grandparents rather than the usual eight. He inherited a range of physical and mental health problems. His shoulders sat at different heights, and a pronounced limp caused by differently sized legs made walking difficult.
He also showed signs of mental instability, including violent outbursts against staff and animals. In one disturbing episode, he forced a member of his staff to eat excrement. He took pleasure in riding his horses so hard that they collapsed with exhaustion and died.
In 1562, Carlos suffered a head injury that was eventually treated with trepanation, a surgery in which a hole is drilled into the skull or parts of it are removed to relieve pressure. After the injury, his outbursts only worsened.
His father denied him opportunities and positions of authority, and his arranged marriages were postponed, fueling Carlos's rage and deepening the rift between them. He allegedly tried to burn a person alive and injured himself to manipulate others.
Carlos hated his father, but his resentment was fiercest toward John of Austria, his popular, illegitimate half-brother. John was successful and favored by the king, and Carlos grew paranoid that John and Philip were conspiring against him.
Carlos secretly plotted to flee to the Netherlands, where a rebellion against Spanish rule was brewing, hoping to gain power by siding with the rebels. He asked John of Austria for support, but John told King Philip everything. Enraged by the betrayal, Carlos tried to shoot John, but the weapon had been unloaded beforehand. He then attacked him with his bare hands, and John survived.
King Philip II personally ordered the arrest of his own son. Carlos was confined to a secure room where access was tightly controlled. All sharp objects, glass, and anything potentially harmful were removed, and even food was delivered under strict conditions.
Rumors spread that he was tortured and that he grew more mad during his imprisonment. He refused food and became increasingly thin, wasting away over six months. In July 1568, Don Carlos was dead at the age of 23. Some said he was poisoned, others that he starved himself, but the modern consensus is that he died of natural causes.
Another example is Ferdinand of Austria, born in 1793 to parents who were double cousins. Double cousins are the children of siblings who marry siblings, meaning they share both sets of grandparents and are related through both parents.
Ferdinand, who was related to the Habsburgs, was born with an inbreeding coefficient of 17 percent, bringing a range of health complications. He was born with hydrocephalus, a condition in which fluid builds up in the brain and increases pressure inside the skull.
In Ferdinand's case, the condition left him with a deformed skull, epilepsy, and a speech impediment. His conditions were not understood, and he was neither encouraged nor stimulated to develop. As a result, he could not pour himself a glass of water, open a door, or climb stairs without help.
Despite this, Ferdinand kept a daily journal of his activities and thoughts and was known for sharp, witty remarks. He was said to be a talented artist with a keen interest in nature and scientific advances, and he could speak five languages.
He reportedly suffered as many as 20 seizures a day, which severely limited his ability to rule. He acted as emperor of Austria in name only, while the day-to-day running of the country was left to others.
In 1831, Ferdinand married Maria Anna of Piedmont-Sardinia, a distant relative. The couple never had children, as Ferdinand was unable to consummate the marriage because of his numerous seizures. Maria often referred to herself not as his wife but as his nurse.
In 1848, a series of revolutions in Austria abolished the absolute monarchy Ferdinand represented. He abdicated and spent the rest of his life in Czechoslovakia, residing at Prague Castle. Retirement suited him well. He wandered the Bohemian streets handing out sweets and money to children and beggars, earning the nickname Ferdinand the Benign. It was the counselors and regents who ruled on his behalf who were blamed for the problems that sparked the revolution. Ferdinand died on June 29, 1885, at the age of 82.
The final example is Maria I of Portugal, sometimes known as Maria the Mad. She was born on December 17, 1734, into the House of Braganza. The royal lineage of the House of Braganza produced 14 monarchs and several inbred marriages.
The family was descended from the Habsburgs, inheriting their lack of genetic variation, yet it continued to marry relative to relative. There were second-cousin marriages, first-cousin marriages, and an uncle-and-niece marriage.
Maria herself married her uncle, Pedro III, her father's younger brother. She was the only surviving child of King Dom Joseph I. Of her eight siblings, four were stillborn, likely because of generations of inbreeding. Since Maria was to be Portugal's first female monarch, she was married to her uncle to solidify her claim and keep rule within the family.
Maria showed signs of mental health problems in her teenage years, suffering bouts of melancholy and nervous agitation. A series of unfortunate events compounded these issues. In 1755, she witnessed the destruction of Lisbon by an earthquake, a tsunami, and fires.
The earthquake struck on All Saints' Day, when the churches were packed, contributing to tens of thousands of deaths. The royal family survived, but Maria doubled down on her religious devotion to avoid a repeat of the tragedy.
Maria married her uncle in 1760, when she was 25 and he was 42. Despite the age gap and family ties, the marriage was by all accounts a happy one. The couple had seven children, but only three survived to adulthood, again likely because of inbreeding.
In 1777, Maria ascended to the Portuguese throne with her husband as regent. Her first nine years were peaceful. She set about rebuilding Lisbon and oversaw the construction of many cultural and religious buildings. Closer relations with Spain were built through marriages between her children and the Spanish royal family.
The king of Spain at the time, Carlos III, was Maria's uncle, meaning cousins were marrying cousins. These cousins were themselves products of inbreeding, compounding the genetic problems. The Spanish princess Carlota, betrothed to marry Maria's grandson, had the infamous Habsburg jaw. As throughout this history, such unions often resulted in more miscarriages, stillbirths, and high rates of infant mortality.
As Maria grew older, she was afflicted more and more by delirium. The first sign came in 1786, when during a routine state function she began screaming that she was burning in hell. Her mental decline worsened after the deaths of her husband, eldest son, daughter, and infant grandson, all in quick succession.
The children had died of smallpox, and Maria blamed herself for refusing to inoculate them. She spent much of her time praying and screaming, believing herself damned and already living in hell. She banned all music and entertainment at court, transforming it into a Catholic mass.
As her mental health worsened, she was subjected to treatments considered pioneering at the time but disturbing today. Diagnosed with melancholia and religious mania, she was treated with ice baths, enemas, and confinement, often bound in straitjackets. One visitor recalled hearing the most agonizing shrieks, inflicting a sensation of horror such as he had never felt before.
Maria was relieved of her duties as queen in 1792 and spent much of her time isolated. During the Napoleonic Wars and the French invasion of Portugal, she and her family fled to their colony of Brazil. Terrified that she was being kidnapped, she screamed all the way to the docks.
In Brazil she earned the nickname Maria the Mad, spending much of her time terrified of the locals. She spent her last years in a convent cared for by nuns, remaining there until her death in 1816, blighted by a mental illness likely caused or worsened by inbreeding.
These cases are just a few of the more disturbing examples of royal inbreeding and its devastating consequences. The desire to maintain power over a kingdom led to a practice that stands as a grim reminder of the dangers of inbreeding.