What is it the theory called when you tell your child that he can't go out an play until he eats his vegetables?

Question

.....very big grin..... Bribery!

Maslow's Theory may be what you are looking for. See the site below.

http://en.wikipedia.org/wiki/Maslow's_hierarchy_of_needs

This would also involve reinforcement (bribery, if you want to use that biased term) and is related to operant conditioning.

http://en.wikipedia.org/wiki/Operant_conditioning

I hope this helps a little more. Thanks for asking.