This chapter is concerned with the effects of intergroup similarity on intergroup attitudes and behaviour. Now similarity can, of course, take a number of forms but, for reasons which will become obvious as the chapter progresses, the discussion concentrates mainly on two. These are attitudinal similarity – that is, similarity between the values and ideologies prevalent in the groups in question, and status similarity – the proximity of groups on some dimension of value or prestige. As I shall show, the concept of similarity occupies a central position in several theories of social relations. Broadly speaking, these fall into two main camps. The first, and by far the predominant view, holds that similarity has attractive or convergent properties on intergroup relations. Thus, groups which are like one another are held to enjoy friendlier and more cooperative relations than those which differ. The second camp reaches quite the opposite conclusion and hypothesizes that groups which become too similar may look for ways to discriminate against each other. The purpose of this chapter is to examine these rival claims in the light of the available evidence.

The discussion proceeds more or less historically. It begins with three major theories dating from the beginnings of modern social psychology – frustration–aggression theory, social comparison theory, and balance theory – and goes on to describe some more recent work in the same mould. These theories collectively make up the first of the two camps – what might be called the similarity–attraction position.