THE extensive empirical literature that has evolved from E. S. Mason's formulation of the Structure-Conduct-Performance paradigm [I3], [14] has been predominantly concerned with the relationship between industrial concentration and profitability. In fact, it may be one of the most frequently tested relationships in economics. Professor Leonard Weiss compiled a table of 46 concentration-profitability studies for the I974 Columbia Law School Conference on Industrial Concentration and referred to another eight in a footnote [35]. Without exception, these studies specify and estimate single-equation models in which causality runs from structure to performance