Internal motivation is longer lasting and more self-directive than is external motivation, which must be repeated reinforced by praise or concrete rewards. Some individuals - particularly children of certain ages and some adults - have little capacity for internal motivation and must be guided and reinforced constantly. The use of incentives is based on the principle that learning occurs more effectively when the student experiences feelings of satisfaction. Caution should be exercised in using external rewards when they are not absolutely necessary; their use me be followed by a decline in internal motivation.

http://ss.uno.edu/ss/teachdevel/Motivat/Motivate.html

That is pretty much what I was trying to say earlier. And shorter, too.

Fear is a key factor in external motivation, there are problems with that, I believe.