Chinny Chin Chin

What happens when your family tree is circle?  Beginning in the 1500s, the Habsburg family set to find out.  For over 100 years, the descendants of Philip I and Joanna of Castille married only among themselves to prevent other families from gaining claims on Habsburg titles.  The process culminated in Charles II of Spain, son of Philip IV and his niece Mariana (this was hardly the only instance of uncle-niece marriage in Charles' ancestry).  Charles' lineage was so intertwined that he was more inbred than the offspring produced by siblings.  This was not good.

The Habsburg line was famous for the pronounced chin that ran in the family, but in Charles this feature manifested as a miserable deformity, as his jaw was so far forward that he could neither chew nor speak intelligibly and drooled uncontrollably.  His handicaps were both physical and mental, and the young Charles did not learn to walk until he was eight years old, and was not capable of receiving any manner of schooling.  He was incapable of being king in every sense, but, since this was medieval Europe, he was king none the less.

Due to the staggering degree to which Charles was unfit to rule, the actual administration of the Spanish Empire fell to the nobles of the court, who spent much of their time jockeying for power and influence while the economy crumbled around them.  Charles' reign (insofar as playing children's games while a country collapses counts as a 'reign') was disastrous for Spain.

The one problem that only Charles could fix was that he was the last of the Spanish Habsburg line, and had to produce an heir or the family would go extinct.  The task of mating with the disfigured drooling imbecile fell to the 16 year old Marie Louise of France (who was at least a mildly distant relative).  She fell into a deep depression and died some years later, without producing any children (it is pretty likely Charles was infertile).  A second marriage to Maria Anna of Neuburg similarly failed to produce offspring.

When Charles died in 1700, the Spanish Habsburgs died with him, and the War of the Spanish Succession was fought to determine who would inherit his crown.  His autopsy report said that his body "did not contain a single drop of blood," which modern scientists consider to be "the stupidest thing I've ever heard."