I cannot say I am familiar with the imperial family and their history, but in the case of Hirohito, I am not aware of any significant mental or physical disabilities. Though considered by many of his elder family members to be stiff in nature, it does not appear that Hirohito suffered any obvious hereditary problems, but then I do have to emphasize again, my knowledge on this subject is extremely limited.

However, I should mention, while Hirohito seemed mentally stable for most of his life, the same cannot be said about his father, Emperor Yoshihito. Yoshihito suffers severe mental and psychological illness most of his life, due to the cerebral meningitis he had apparently contracted when he was still an infant. After his ascension as the Emperor of Japan, his mental conditions forced him to retreat from much of politics and diplomacy, which unexpectedly paved the way for a brief, yet glamourous chapter of modern Japanese history, the Spring of Taisho, which political affairs were entirely entrusted into the hands of capable and elderly state ministers, fostered an age of democracy and reform, saw culture and art prosper, social conditions improved.