Intrinsic vs Extrinsic Motivation
Say I ride my bike just for the enjoyment of riding my bike, this is an intrinsic motivator. I am performing the task for the sake of performing the task.
The opposite is an extrinsic motivator, where I perform a task for a reward. For example, working at a job for a paycheck.
The Overjustification Effect
When someone who is intrinsically motivated to perform a task is given a reward for performing that task, they lose intrinsic motivation. They are less likely to be motivated to perform the task in the future if not for a reward.
Researchers had some students draw pictures for a reward, and had other students draw pictures with no incentive to draw whatsoever. The students who were given a reward were then half as likely to continue drawing afterward, and their drawings were worse. The reward cut the students motivation to draw for the sake of it.
Conversely, a third set of students were given a reward unexpectedly after having drawn drawn with extrinsic motivation. These students were even more likely to keep drawing than the purely intrinsic group. The surprise reward boosted the motivation to draw for the sake of it.