As schools around the country wrap up their first month back in session, parents soon may start receiving the first reports on how their children are shaping up — literally.

Public schools in 19 states now track students’ body mass index numbers and report the fat metrics back to parents. There may, indeed, be more schools tracking students’ BMI than there are schools teaching kids the arithmetic needed to do such calculations on their own. To some, this would seem like the prudent step to take given that childhood obesity has nearly tripled since 1990. The Centers for Disease Control and Prevention says about one in five American children is obese, and one in three is overweight or obese.

There are only a couple of problems with this increasingly popular nanny-state tactic: There’s not much evidence that BMI-tracking reduces obesity, and it may harm the very children it’s meant to help.

My latest, for the R Street Institute. Read the whole shebang.