According to an old book, "Koji Dan," Emperor Sutoku was not Emperor Toba's real child but the child of Emperor Toba's grandfather, Emperor Shirakawa, and Tai ken mon in, since Tai ken mon in had been passed from Emperor Shirakawa to Emperor Toba.
古書『古事談』には、これは崇徳天皇が鳥羽天皇の実子でなく、鳥羽天皇の祖父白河天皇と、白河が後に鳥羽に与えた待賢門院との間にできた子であったためとある。