The tendency for people to see members of an out-group as less diverse and more stereotypic than the members of that in-group see themselves is called .

Out-group homogeneity effect

Explanation:

Out-going homogeneity effect is defined as the approach and realization persisted by  person that out-group members are more similar in comparison with in-group members of particular group.

This  tendency is also known as out-going homogeneity bias.Beliefs and thinking in this effect makes people believe that out group members are more alike and show less diverse traits.