Affinity in terms of sociology, refers to "kinship of spirit", interest and other interpersonal commonalities.
Social affinity is generally thought of as "marriage" to ideas, ideals and causes shared by a tight community of people.
In Social affinity in a modern world, Boston College professor, James Allan Vela-McConnell explores the emergence of the concept of "social affinity" bridging classical sociology and social psychology, identifying "the notion of social cohesion" based upon the sentiment of moral obligation.
[2] Affinity is shown or demonstrated by an individual identifying with a subculture, ethnicity, or other groups, within a larger national culture.
Self-identification with a group is a valid form of expressing affinity.