WHAT are the boundaries that separate the social science disciplines from each other? Where (if anywhere) do we draw the line between those problems that call for the expertise of the economist and those that demand the skills of the sociologist, or the psychologist, or the political scientist? This is not the kind of methodological question that interests only those scholars who don’t want to get on with the job: it is at the center of some debates in Washington about past and prospective decisions on public policy, and has an inescapable intellectual importance as well.