In the English language, black sheep is an idiom used to describe an odd or disreputable member of a group, especially within a family. The term stems from the genetic effect in sheep whereby a recessive gene occasionally manifests in the birth of a sheep with black rather than white coloring; these sheep stand out in the flock.
The term has typically been given negative implications, implying waywardness. It derived from the atypical and unwanted presence of other black individuals in flocks of white sheep.
In psychology, the black sheep effect refers to the tendency of group members to judge likeable in-group members more positively and deviant in-group member more negatively than comparable out-group members.