Somewhere along the lines, a long time ago, ruling families got the idea that their blood was better than everyone else's, so they wanted keep reproduction in the family, so to speak. This was also a convenient way to keep inherited wealth from being divided. But inbreeding will catch up with you sooner or later, as more and more harmful genes have the chance to get doubled up. The case we are most familiar with is that of Charles II of Spain, whose family tree was not only a wreath, but even more resembled an Euler diagram.
But he was far from the only victim of royal inbreeding. It's always been common in Europe. Loony Throne takes us through several dynasties in which inbreeding made a lasting mark on history. It would have been far healthier to accept the children of these kings' and queen's extramarital relationships with commoners, before their bloodline crashed into a wall. The exception is Queen Victoria's family. In that case, the harm wasn't a confluence of inbreeding leading down to one last tragic ruler, but in her scheme of marrying off her children and spreading a genetic disease to other ruling families of Europe.
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