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There was a king of Spain whose jaw stuck out so far his teeth couldn't meet. His tongue was so big he could barely talk. He had seizures. He looked like an old man at 30 and died childless at 39. His entire family had been marrying their cousins for nearly 200 years.
His name was Charles II, the last of the Habsburgs. And he is the textbook case scientists still point to when explaining why countries ban cousin marriages.
The Habsburgs ran most of Europe in that period, and they refused to marry outside the family. Uncles married nieces. First cousins married each other. Royals kept marrying back into the same bloodline, generation after generation, to keep the crown in the family.
Spanish scientists eventually pieced together the family tree to see what this did to their DNA. They tracked 3,000 people across 16 generations. They calculated something called an inbreeding score. It measures how much of your DNA ends up in two identical copies because both your parents inherited it from the same ancestor.
A normal person scores almost zero. Philip I, the dynasty's founder, scored 0.025. Charles II, born about five generations later, scored 0.254. That is the same score a child would get if their parents were brother and sister.
Every person quietly carries a few broken genes. They usually don't cause problems, because the matching copy from your other parent works fine. But when both your parents come from the same closed family tree, you start inheriting the same broken copies from both sides. Nothing cancels them out. They start showing up.
About a quarter of Charles's DNA was identical pairs of these broken genes, which is why his body fell apart. He couldn't have children. When he died in 1700 with no heir, every kingdom in Europe started fighting over who would inherit Spain. The war that followed killed an estimated 700,000 people.
A UK study followed 11,000 babies and found the same pattern. Children born to first cousins were twice as likely to have a birth defect, 6% versus 3%.
Norway banned cousin marriage in 2024. Sweden's ban takes effect July 1, 2026. The Habsburgs ran this experiment over three centuries ago, and the science has been clear ever since.
Visegrád 24@visegrad24
Sweden has just BANNED cousin marriage and close relative marriage! 🇸🇪
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