Step 1: Understanding the Concept:
The question asks to identify the sociologist who introduced the fundamental classification of social groups into 'in-groups' and 'out-groups'.
Step 2: Detailed Explanation:
The distinction between in-groups and out-groups was developed by the American sociologist William Graham Sumner. In his influential 1906 book, "Folkways", he defined these terms:
- In-group: The group to which an individual belongs and identifies with, characterized by a sense of loyalty, solidarity, and "we-feeling".
- Out-group: A group to which an individual does not belong, often viewed with a sense of indifference, competition, or even hostility.
This concept is central to understanding group dynamics, identity, and intergroup relations.
Charles Horton Cooley, another sociologist listed, is known for classifying groups into primary and secondary groups.
Step 3: Final Answer:
William Graham Sumner divided groups into In-group and Out-group.