Joel's older brother still wearing his varsity jacket from when he played baseball at Miller High School is an example of the component of group dynamics known as "Group Identity and Cohesion."
This behavior demonstrates a sense of belonging to a specific group (in this case, the baseball team) and a strong connection to the group's identity, which is often symbolized by items like varsity jackets. It shows that group membership and the associated identity continue to be important to Joel's older brother, highlighting the lasting impact of group dynamics on individuals.