When you encounter an adult who has difficulty getting along with others and maintaining any relationships, you will probably be tempted to consider how he or she was treated in his or her first group.

secondary
reference
primary
aggregate

The answer is the primary group.

Explanation:

In a primary group, the different members usually share close, personal, as well as enduring relationships. In these groups, people usually share activities and cultural aspects. Some examples of a primary group include family or childhood friends.

The term was established by Charles Cooley through his book Social Organization: A Study of the Larger Mind. It is said that primary groups are very relevant concerning the development of personal identity.