I recently wrote about research finding benefits of teacher merit pay in South Carolina and Texas. Yet bias in teacher evaluations is a real problem, complicating performance pay systems. “Teachers with higher-achieving and less disruptive students, holding constant the teacher and school, receive systematically higher performance ratings,” concluded a pair of researchers in a recent study. Relatedly, a new study finds that Texas’ merit pay system has encouraged some teachers to move to more-advantaged schools.