The real part of the experiment came 2 weeks later. During playtime the drawing tools were put out in the room. The children weren’t asked anything about drawing, the tools were just put in the room and available. So what happened? Children in Groups 2 and 3, the Unexpected and the Control Groups spent the most time drawing. The children in Group 1, the ones who had received an expected reward, spent the least time drawing. “Contingent” rewards (rewards given based on specific behavior that is spelled out ahead of time) resulted in less of the desired behavior. Later the researchers went on to do more studies like this, and with adults as well as children, finding similar results.

https://www.blog.theteamw.com/2011/03/15/100-things-you-should-know-about-people-81-intrinsic-rewards-trump-extrinsic-rewards/