One thing royal families do have more of than the rest of us: genetic defects. People of noble heritage are called blue bloods, and for hundreds of years the ruling families of Europe have married almost exclusively within their pre-existing noble lines in an effort to keep the bloodline "pure." The family trees are incredibly complicated and tangled. Marrying into another royal family didn't help, because not only were the other families equally inbred, but the families had intermarried so much in the past that the same genes were being passed around for generations.
This is a problem because interbreeding creates the strong potential for the expression of recessive genes. Recessive genes aren't expressed unless two copies are passed to a child. So if someone with a recessive gene marries someone from another family, the spouse is far less likely to carry the same recessive gene. However, people in the same family have similar genes, and often carry the same recessive genes. One result -- Queen Victoria, who passed along a tendency toward hemophilia to generations of British royals. Royals have a long history of mental deficiencies, insanity and other congenital defects [source: Shaw].