Clear rules of royal succession made kings less likely to be murdered
Who inherits a throne sounds like a question of family trees, but it shapes whether a state survives its ruler's death. Research links primogeniture to fewer regicides: when only one person can step up, there is far less to gain from killing the monarch. Europe's rules have been quietly rewritten since 1980.
An order of succession fixes who takes over a high office or title when it falls vacant, by descent or by law. Elected systems use it too, so an office never sits empty while a vote is organised. The details vary: when India's presidency is vacant, the vice president only carries out the duties until a new election, whereas in the Philippines the vice president simply becomes president for the rest of the term. Bodies with no rule at all need succession planning to avoid a power struggle.
For monarchies, predictability brings more than safety. Studies find authoritarian regimes built on primogeniture more stable than those with other arrangements, and a known heir can be trained, educated and protected from childhood. Some dynasties hedged their bets by crowning the heir during the old king's lifetime, as with Henry the Young King, while Gaelic tanistry elected the heir from eligible royal men.
The rules themselves differ sharply. Under male-preference primogeniture, used in Spain and Monaco and long in Britain, sons come before daughters, though a woman can inherit if she has no brothers or brothers' heirs. Salic law shuts women and their descendants out altogether, and still governs Liechtenstein and Japan's Chrysanthemum Throne. Disputes over which rule applied could be explosive: in 1830 the question of whether Ferdinand VII's daughter Isabella or his brother Charles should follow him led Spain into a series of civil wars and spawned a rival line of pretenders.
The modern trend is absolute primogeniture, in which the eldest child inherits regardless of sex. Sweden adopted it in 1980, followed by several neighbours, and the United Kingdom and Commonwealth realms switched in 2013 for anyone born after 28 October 2011. Some Indian kingdoms had long allowed a childless king's widow or mother to rule in her own right, and Razia Sultana of Delhi succeeded her father even though her brothers were alive.
Source: Order of succession
