The existence of social differences in educational achievement as a social fact presents the sociology of education with a challenge to which it has responded with indifferent success. It is argued that contemporary explanations that dismiss the existence and relevance of differences in cognitive performance arising as a consequence of class variation in socialisation are likely to misrepresent the real causes of inequality/difference. The substantive discussion, organised around six questions dealing with the explanatory capacity of this concept, suggests that a satisfactory theory of inequality of educational opportunity will need to concern itself with the effects of socialisation on cognition. Some implications for educational practice and policy-making are briefly noted in conclusion.