Yano, Akiko (2013). "Historiography of the "Phallic Contest" Handscroll in Japanese Art". Japan Review (26): 59–82. JSTOR41959817. The pertinent discussion is on pages 59-61, citing Kokon Chomonjū (completed in 1254) as the source text that makes the connection between Toba Sōjō and the Phallic Contest theme.