VADUZ, Aug. 16 -- The man who controls one of the last genuinely powerful monarchies in Europe stepped before his nation's celebration on Friday and announced he would, in time, give up the principle that has governed royal inheritance in Liechtenstein for centuries.

Hereditary Prince Alois, who runs the 40,000-person principality on the Rhine while his father, 80-year-old Hans-Adam II, retains the throne in name, revealed that the ruling family had voted two days earlier to end male primogeniture. The first-born child of the succession line, regardless of sex, will now become the heir. "The line of succession will be adjusted from male primogeniture to absolute primogeniture," Alois said during a national day speech, "so that the first-...