The odds against a boy may be a little stronger than 50-50 since girls seem to run in the family. He has two children, both girls. The prohibition is, after all, not ancient, being first promulgated in the 19th century. Japan has had reigning empresses before. The most recent was Go-Sakuramachi, who kept the throne warm from 1762 to 1770, then abdicated in favor of her nephew.
Opinion polls show people are ready for change.
All European royal houses allow for female succession, a few permitting succession by the first-born regardless of sex. The Japanese are a conservative people, especially when it comes to the imperial family, so change will come slowly. But they are practical too. If Masako grows older and the chances of her becoming pregnant, let alone having a son in her first pregnancy, grow slimmer, the Japanese will do what they have to do to keep the institution alive. All Rights Reserved.