Lewis Morgan became fascinated with ancient cultures, and then with the Native American societies. He became close to the Seneca, making a close friend of Ely S. Parker studying the Iroquois culture in great detail as well as helping them in their legal battles over rights to their land. Thus, his understanding of their culture was quite authentic.
Yet, Morgan was a proponent of cultural evolution. He proposed a unilinear scheme of the evolution of human society from primitive to modern, through which he believed all societies progressed. Thus, his study of different societies was always in comparison with his own, more "advanced" society. In the area of technology, this view was rather helpful, as he was able to categorize levels of societal development based on their advance beyond relying on the environment to supply their needs (hunter-gatherer), to simple agriculture and more advanced tool production to provide the basics but without the refinements and sophistication of social relationships in modern society (barbarism), to the advanced stage of "civilization" in which culture could not only develop but be recorded and transmitted through written forms. Although taken up by Karl Marx and Friedrich Engels in their theories, Morgan's own view was not as materialistic as their interpretation of his work appeared.
Morgan studied many different aspects of society, including ceremonies, religion, and politics, as well as kinship and marriage. Starting with his detailed study of the Iroquois he recognized the significance of kinship relationships and marriage in society. Through comparative studies of this aspect, he developed an evolutionary model of family structure, which entailed development from a "promiscuous horde" with no real family structure, through stages in which incest and group marriage were practiced to those in which polygamy was the norm, to what he considered the most developed stage—monogamy. Although his evolutionary stages have been rejected, his recognition that marriage and kinship relationships are fundamental to society played an important role in the development of anthropology.