The reason Nobutada was thought to be foolish was due to an account of the incident of Nobuyasu MATSUDAIRA's seppuku (suicide by disembowelment) related in the "Seishi Tanko" which was written by Mitsutoshi TAKAYANAGI.
信忠が暗愚とする根拠は、高柳光寿の著書『青史端紅』において松平信康切腹事件の真相について語られた説に由来する。