Why did European royal families often arrange marriages between relatives?

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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

European royal families frequently arranged marriages between relatives, known as consanguineous marriages, primarily to consolidate power, wealth, and prestige within their dynasties. By keeping marital alliances within the family, royals aimed to ensure the security of their lineage and maintain control over their territories.1

The House of Habsburg serves as a prominent example of this strategy, famously summarized by their motto: "Let others wage wars: you, fortunate Austria, marry!". While these unions were effective at preserving political influence for centuries, they often resulted in severe biological consequences. Generations of inbreeding led to the inheritance of genetic abnormalities, such as the distinctive "Habsburg jaw" and increased rates of infant mortality. Other royal lines, including descendants of Queen Victoria and Prince Albert, suffered from hereditary conditions like hemophilia.1

Ultimately, these marriages were rarely motivated by personal affection; they were calculated political maneuvers designed to keep power concentrated among a select group of relatives.1