More to Back Up Positive Reinforcement

Here's a paragraph from my psychology book. This time it tells you why positive reinforcement increases the desired behavior and negative reinforcement increases the unwanted behavior.

"In one experiment some elementary school children were told in the course of classroom activity that they were neat and tidy (group A); others were told that they should be neat and tidy (group B); and still others were given no special treatment (group C). The result was that those in group A condition became significantly neater, as measured by the absence of littering, than did those in either of the other conditions. Similarly, children who were told that they were good at math showed greater subsequent improvements in math scores than did those who were told that they should try to become good at math. In these experiments the change in behavior presumably occurred because of a direct effect of the appraisals on the children's self-concepts, which they then strove to live up to."

How can I knock this into my brain? Treating a person like they are already doing the desired behavior is what changes behavior; not criticism and nagging. As you can see my "world" is falling apart here. I'm known as the nagger around town (actually just by one person).