Introduction: peer influences on learning: where are they?

This introduction sets the stage for the themed issue. It describes the ambiguities inherent in the interpretation of the literature on compositional and peer effects and the genesis of the project. The balance of the introduction then describes the approach adopted to search for peer effects. First, there is a brief outline of the Barr and Dreeben (How Schools Work, University of Chicago, Chicago; Sociol. Educ. 61 (1988) 129) model as a conceptual heuristic to aid in the search for peer effects. Second, a distinction is made between compositional and peer effects. Third, there is description of the evidence used to identify compositional effects in the empirical literature and the role of theory and data in determining whether these effects implicate peers. The introduction concludes with an outline of the organization of the themed issue.