The heads of rival nations during World War I, a conflict in which more than eight million soldiers died, King George V of England and Kaiser Wilhelm II of Germany were first cousins, both grandchildren of Queen Victoria.

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Why Have So Many World Leaders Married Their Cousins?

A crown. A ring. Your cousin. What else do you need for a royal wedding? At this point, the fact that so many royals through history wed relatives in what are now termed consanguineous marriages is something of a history joke. But why did they do it in the first place? Before we begin, an