The precursor to the theory was first conceived by three Japanese historians Shigeno Yasutsugu, Kume Kunitake, and Hoshino Hisashi with the publication of Kōhon kokushi gan (稿本国史眼) prior to the annexation.

[2] The book asserted that the legendary figures Susanoo, the brother of Emperor Jimmu, Inahi no Mikoto and Empress Jingū had ruled or invaded ancient Silla (Korea).

To counter this, historians such as Shiratori Kurakichi, founder of the discipline of Oriental History (Tōyōshi/東洋史) in Tokyo Imperial University argued that the Korean deity was fabricated by Buddhist priests sometime after 372 CE.

[5] This allowed him to demonstrate "that Korea as a unified country developed relatively late in the history of Asia, and later than Japan",[8] ultimately discrediting Dangun's supposed accomplishments in favor of the Japanese god.

[11][12] Hirata Atsutane was among those who used their studies of Kojiki and Nihon Shoki to claim that Korean and Japanese history was intertwined from the period of ancient nation formation and that a hierarchical relationship in which Japan was dominant could be established.

The Meiji period historians Hoshino Hisashi, Kita Sadakichi, and linguist Kanazawa Shosaburo have been criticized for promulgating theories of common ancestry used to justify Korea's annexation and policy of cultural assimilation.

The theory gained further momentum among historians with much emphasis put on the natures of both the Japanese and the Koreans by comparing the two ethnic groups to Amaterasu and Susanoo as siblings with shared heritage but vastly different personalities.

[14] There was also an ancient view that Ōkuninushi was a foreign deity from the Korean peninsula, which was also later used as justification for Nissen dōsoron, although there is no evidence for this beyond the Kojiki and Nihon Shoki listing him as a descendant of Susanoo.

Nissen dōsoron was the "main pillar" supporting the related concept naisen ittai (内鮮一体),[19] or unity of the inland (内) with Korea, which was represented with the character 鮮 (sen), derived from the name Chōsen.

[28] This forced the ethnic Koreans to become a target of hostility and animosity which in turn made Nissen dōsoron now a blasphemous theory and an obsolete piece of history for the Japanese.

[29] Evidence for both Northern and Southern mtDNA and Y-DNA haplogroups has been observed in the Japanese, with the North-Eastern DNA taking up majority of the genetic makeup,[30] especially among the Mainland group.

[31] In addition to the Northeastern ancestry, the Japanese demographics (alongside the Koreans), are the only ethnicities to have restricted presence of the Jōmon-like M7a DNA in East Asia.

[32] Though being a controversial theory for both nations, contemporary historians have directed to certain historical accounts that could suggest a close relationship between the Japanese Imperial family and the Korean peninsula.