Rus' people
Kostomarov was a specialist of East Slavic folklore. He put forward the idea that there are two types of "Rus' people", those of the Kievan background, among the Dnieper Basin, which he called Southern Russians, and those of the Novgorodian background, which he called Northern Russians. Kostomarov observed Northern Russians as a political hegemon of the Russian state. As a historian, Kostomarov's writings reflected the romantic trends of his time. He was the first Russian historian who used of ethnography and folksong in history, and tried to discern the "spirit" of the people, including the so-called "national spirit", by this method (see about: , narodnost'). On the basis of their folksongs and history, he said that the peoples of what he called "Northern" or "Great Rus on one hand and "Southern" or "Little Rus on the other (Russians and Ukrainians, respectively) differed in character and formed two separate Russian nationalities. In his famous essay "Two Russian Nationalities" (), a landmark in the history of Narodniks thought, he wrote what some consider to be the ideas of Russians inclined towards autocracy, collectivism, and state-building, and Ukrainians inclined towards liberty, and individualism. The article of Kostomarov on the problem of the psychological diversity of Rus' people in the Russian Empire had an impact on the scientific research of the collective psychology in Eastern Europe.