Though exceptions undoubtedly exist, women with higher aptitudes can ordinarily be expected to be more effective classroom teachers than those with lower aptitudes. It is therefore troubling to think that in the United States those entering the teaching profession in recent years have, on average, lower measured aptitudes than their predecessors. That able women are no longer entering the teaching profession at anywhere near the same rate as in the past is of special concern, since women compose approximately 75 percent of all elementary and secondary school teachers, almost the same percentage as 40 years ago. This article describes a study conducted by the authors on what accounts for the decline in teacher quality. Data from a series of surveys conducted by the federal government for various years between 1961 and 1997 were analyzed. The factors contributing to the reduced likelihood that women of high aptitude will enter the teaching profession appear to come from both within and outside the teaching profession. The authors focus on two that can be expected to be of critical significance. First, within the teaching profession, the pay scale of public school teachers has become increasingly compressed since the 1960s. Second, outside of teaching, college-educated women have achieved greater parity in their pay vis-a-vis male workers, luring more able women to alternative professions. After systematically comparing the relative importance of the two factors, they found, surprisingly enough, that pay compression within the teaching profession, induced by the introduction of collective bargaining, has had by far the greater effect. (Contains 4 figures.)