As everywhere in Europe in the past, royal property was inherited. If one of the members of the royal family were to be married to someone "from the side", in the event of a split, a huge territory would have to be divided, which often included entire states and peoples.
The simple solution of the Habsburg dynasty for this "problem" was to marry each other!
During their reign, the Habsburgs made at least 80 percent of marriages with close blood relatives. What was not known at that time was the fact that the marriage of close relatives could have consequences for the descendants. The dynasty became known for a high percentage of stillborn children, and those who would have survived were not entirely "normal."
One of the deformities that was the most common among the descendants of cousins became known as the "Habsburg jaw", and all the horror of crossing close relatives can be seen today in the portraits of members of this dynasty.
The circuit of genetic mutations eventually broke on this ruler - King Carlos II of Spain.
In Spanish history, Carlos II is known as the "Enchanters". He simply had so many psychophysical disorders that the people were convinced that some supernatural forces were at work. Namely, Carlos's branch of the Habsburg lineage had read through cases when sisters gave birth to children from marriages with their uncles. Carlos' mother was the niece and wife of his father and daughter Maria Anne of Spain and Emperor Ferdinand III. Thus, Maria Ana was Carlos's aunt and grandmother at the same time, and his mother was his uncle's sister at the same time.
In 1679, eighteen year old Carlos married his cousin Marie Louise of Orleans. They had no children because Carlos was impotent and infertile, but the medicine of that time, according to the established custom, blamed the queen exclusively for the lack of descendants. Maria Louise became depressed and died at the age of 27 after ten years of marriage.
Due to the necessity to get an heir the following year, Carlos married another cousin, Maria Anna of Neuburg. There were no children in this marriage either.
The practice of incestoid marriages took its greatest toll, and the rule of the Habsburg dynasty in Spain was extinguished by the death of Carlos II.
Disease definitely