Intergroup bias is a cognitive bias in which people favour members of their own social group over members belonging to other social groups. This favouritism can be expressed in many ways, such as by positive evaluation and the allocation and sharing of resources. The origin of the term “inter-group bias” was coined by the sociologist William Sumner who, in 1906, stated that humans, as a species, join together in groups by nature, and have an innate tendency to favour members of their own group over others.

Source:
Sumner, William Graham (1906), “Folkways: a study of the sociological importance of usages, manners, customs, mores, and morals“, Boston: Ginn and Co.