motivation

Rewards can destroy your love for a hobby.

A 1973 study showed children offered a reward for drawing spent significantly less time drawing during subsequent free-play sessions.

The study measured the duration of engagement (percentage of time spent drawing), not the binary likelihood of children choosing to draw.

Your desire wanes when you treat your interests like work.

Where this comes from

Study: Lepper, Greene, and Nisbett, 1973
What the check found: The original paper found rewarded children spent about 8% of their time drawing vs. ~16-17% in other groups. Framing this as '50% less likely' is a common but imprecise statistical shorthand.
Confidence: Solid, with limits

One of these every day, and every one names the study it came from. When a finding is contested, it says so.