In 2017, the National Diet changed the law to enable the Emperor Akihito to abdicate within three years.
The new law further restricted the succession to legitimate-born sons, grandsons, and male line descendants of an Emperor.
Previously, an Emperor's sons and grandsons born by concubines and their male line descendants could succeed to the throne.
Although Imperial chronologies include eight reigning empresses in the course of Japanese history, their successors were most often selected from amongst the males of the paternal Imperial bloodline, which is why some conservative scholars argue that the women's reigns were temporary and that male-only succession tradition must be maintained in the 21st century.
In addition, the law contained numerous mechanisms to regulate the future size of the Imperial Family, and thus the financial burden on the state.