The Japanese parliament passed a historic amendment to the 19th-century Imperial Court Law stipulating that only men of paternal descent can become emperors.
The amendments include adopting distant male relatives to produce future heirs, and allowing princesses to retain their royal status after marrying commoners.
Emperor Naruhito's 24-year-old daughter is immensely popular, and many Japanese want her to succeed him. However, Princess Aiko is ineligible because she is a woman.
Japan's male-only succession rule means that the emperor's teenage nephew and second in line to the throne, Prince Hisahito, is likely to be the heir.
