Teachers matter--and some matter more than others. That recognition has driven a tidal wave of controversial policy reforms over the past decade, rooted in new evaluation systems that link teachers' ratings and, in some cases, their pay and advancement to evidence of classroom practice and student learning. Two out of three U.S. states overhauled…

Experienced teachers are, on average, more effective at raising student performance than those in their early years of teaching. This gives rise to the concern that too many teachers leave the profession after less than a full career and that too many leave troubled inner-city schools for suburban ones. Until now, the roots of these problems have…