What is the black sheep effect?

According to the black sheep effect, someone who is socially undesirable is liked less if they belong to our group (an ingroup member) than if they belonged to a different group to which we do not belong (an outgroup member).

The black sheep effect is a form of ingroup bias or ingroup favoritism: by derogating unlikable ingroup members, people protect (and thus promote) the positive image and identity of the ingroup as a whole.