Overjustification effect
Also said as Undermining effect, Crowding out
Rewarding people for something they already enjoy can make them do less of it once the reward stops. They come to see the reward as the reason they were doing it.
It warns you that bolting rewards onto something already fun can cost you. Its scope is argued over. Lepper, Greene and Nisbett showed it in 1973 in preschoolers expecting an award for drawing. Deci, Koestner and Ryan's 1999 review of 128 experiments found tangible, expected rewards do undermine interest, while Cameron and Pierce argued the harm is limited and avoidable. Surprise rewards and praise did not show the same harm in those reviews.
Say it
“Do not pay out coins for finishing the story chapters. People are already reading them because they want to know what happens. Give a surprise gift at the end instead of a promised reward up front.”
Not to be confused with hedonic adaptation
Adaptation means a reward stops feeling good because you are used to it. Overjustification means the reward changes why you think you are doing the activity.