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Why was Charles II of Spain’s inbreeding coefficient especially high among the Habsburgs?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

Charles II of Spain’s exceptionally high inbreeding coefficient of 0.254 was the result of the House of Habsburg’s long-standing strategy of marrying within their own family to consolidate power and prestige.12

While the average Habsburg had an inbreeding coefficient of 0.093, Charles II’s significantly higher figure reflects the culmination of generations of this practice. To maintain their vast territorial holdings and prevent rival dynasties from gaining influence through marriage, the Habsburgs frequently wed members of different branches of their own family. This systematic intermarriage was so central to their political strategy that it was captured in their famous motto: "Let others wage wars: you, fortunate Austria, marry!"12

Ultimately, this pursuit of dynastic preservation led to severe biological consequences, including the "Habsburg jaw" and other genetic abnormalities, which contributed to the decline of the dynasty and the eventual extinction of the male line of Charles V.12