Several European heirs are already female, including Crown Princess Victoria of Sweden, Princess Elisabeth of Belgium, Princess Leonor of Spain and Princess Catharina-Amalia of the Netherlands. Britain and other Commonwealth realms adopted absolute primogeniture via the Perth Agreement in 2011, affecting descendants like Princess Charlotte.
By contrast, Japan recently approved a bill to allow distant male relatives over 15 to be adopted into the imperial line — the first main change to the Imperial House Law since 1949. The law still bars women from inheriting, so Emperor Naruhito’s daughter, Princess Aiko, remains ineligible; the line continues through his brother Fumihito and Fumihito’s son, Prince Hisahito.