This was a great proof-of-concept piece by Wansink et al to show that there are simple, inexpensive, and relatively unobtrusive means of "nudging" individuals towards healthier choices.
Though I understand that this was a preliminary study, I hope that in subsequent studies the authors consider the possibility that the improved choices were made because the children knew they were being measured. It is possible that the parental discussions (which were highly variable and uncontrolled in type/frequency/etc.) had little impact. Another study group in the future can include children who knew they were being measured, but no report cards were sent (or were only shown to the children themselves).
It has certainly been shown before that even in the absence of external pressure (in this case, potentially being spoken to by each child's parents), the mere internal pressure (or at least the possibility of external pressure even without it occurring) can be just as powerful an element.
I applaud the authors on their work, and I look forward to hearing more from them.

Peter Henderson, MD MBA
Department of Surgery
NewYork-Presbyterian/Weill Cornell Medical Center