First, be found diminishing returns. In this study, Monk discovered that, for teachers of high school juniors, the number of college content courses teachers take makes a difference in the achievement of their students--but only up to a point. After five courses in mathematics and four courses in the physical sciences, additional courses had little effects on student outcomes. (A number of studies have reported on effects, or even negative effects, for advanced study of subject matter--as if advanced study creates compressed froms of knowledge that distances teachers from the content they teach and from the struggles of their students.)