The "Nihonshoki" describes that he was the Kotaitei (the younger brother of an Emperor so is heir apparent) of his older brother the Emperor Tenchi to begin with; however, some say that the book tries to deny the fact that the Emperor Kobun actually was the investiture of the Crown Prince, and justifies his succession to the imperial throne.
また同書では当初から兄・天智天皇の皇太弟であると記されているが、弘文天皇の立太子の事実を否定し天皇の皇位継承を正当化する意図があるとする意見もある。