Overjustification:

Dissonance theory
http://darkwing.uoregon.edu/~bfmalle/sp/L7.html

Paradigm of overjustification:

Lepper, Greene, & Nisbett (1973)--in the reader. In this situation, children who already like to draw
receive an additional reason why they do it ("overjustification"), namely, an award. Under this new
self-interpretation, they don't draw anymore because they like it but because they want they award; as a
result, their drawing is less fun (and less good).

The "Overjustification " Hypothesis:
A person’s intrinsic interest in an activity may be decreased by inducing them to engage in the activity as an explicit means to some extrinsic goal.