In premodern Japanese, it was traditional for a prestigious or extremely honorable man whose name was read with kun'yomi to have the reading of his personal name changed to on'yomi in certain situations, a process called 有職読み【ゆうそくよみ】. (Women virtually never had 有職読み, presumably because Heian period women were not supposed to read kanbun.) Some notable examples of this include:
- 源 頼光 Minamoto no Yorimitsu (944-1021), who became Minamoto no Raikô
- 徳川 慶喜 Tokugawa Yoshinobu (1837 - 1913), who became Tokugawa Keiki
- 伊藤 博文 Itô Hirobumi (1841 - 1909), who became Itô Hakubun
- 原 敬 Hara Takashi (1856 - 1921), who became Hara Kei
I have read that 有職読み applies especially after the individual's death. However, Wikipedia doesn't bear this out and it also has some odd counterexamples:
- 野沢 那智 Nozawa Nachi (1938 – 2010) was asked whether his personal name's real reading was Yasutomo and said that he wasn't sure.
- 横溝 正史 Yokomoto Seishi (1902 – 1981) had a kun'yomi on his birth certificate but it was never used.
So, two questions I'd like to have verified: is there a general rule by which the use of on'yomi is applied, and has it ever applied to people born after 1945? Is it entirely arbitrary, or was there a rule that was abandoned at some point?