It has been believed that the reason why Yasutoki studied "Hososhiyo-sho" (a legal book compiled by the Sakanoue clan between the end of Heian period and the early Kamakura period) as well as the criterion of Myobodo (study of Codes) (legal documents of petition, petition for objection, and so forth) and why his uncle Tokifusa HOJO, a bakufu rensho (assistant to regents in bakufu) who had worked as Rokuhara Tandai (an administrative and judicial agency in Rokuhara, Kyoto) with him and person who had come from lower-ranking government official in Kyoto and their descendents participated in the compilation were that he sought the source of law not for the common law for samurai families, but for the court noble law.
これは泰時が『法曹至要抄』や明法道の目安（訴状・陳状などの法律文書）を研究していたことや、編纂に参加しているのが泰時とともに六波羅探題を務めた叔父の北条時房（幕府連署）や京都の下級官人出身者やその子弟が中心であったのも御成敗式目の法源を武家慣習法ではなく公家法に求めたからと考えられている。