Substantial political power is often attributed to interest groups. The origin of this power is
not quite clear, though, and the mechanisms by which influence is effectuated are not yet
fully understood. The last two decades have yielded a vast number of studies which use
empirical models to assess the importance of interest groups for the formation of public
policy. Each of these studies yields insights on particular, confined aspects of interest group
politics. To get a more complete picture of the results, however, a broad survey of the …