In his review of episode eight of The Wire, Craig Jerald at The Quick and the Ed writes that "American teachers themselves have a much shallower grasp of math concepts than Chinese teachers, despite spending many more years in school to become a teacher" and cites the research of Deborah Ball and others.  However, in the American Educator article referenced by Jerald, Ball identifies the real problem, that "the mathematical knowledge of most adult Americans is as weak, and often weaker" than that of teachers.  Writes Ball,

We are simply failing to reach reasonable standards of mathematical proficiency with most of our students, and those students become the next generation of adults, some of them teachers.  This is a big problem, and a challenge to our desire to improve.

Ball believes that to improve mathematical proficiency, we must identify and bolster the "mathematical knowledge for teaching" which is not merely content knowledge, but "specialized knowledge for teaching mathematics" that is independent from "common content knowledge--the basic skills that a mathematically literate adult would possess."  Jerald's post glosses over what Ball means by mathematical knowledge.

Ball is also quick to point out why identifying the "mathematical knowledge for teaching" matters; early evidence suggests that improving teachers' knowledge will enhance student achievement.  The AFT-affiliated Shanker Institute is supporting Ball in this work.  And readers may also want to look at another American Educator article by Richard Askey, which discusses Liping Ma's work, a researcher who is also mentioned in Jerald's post.