Found via @dgilmour.
50 years ago, Edward Deci gave different groups of students a Soma cube puzzle to solve. Some were paid to take part, others weren’t. When he announced that the time was up, the students that were paid to work on the task just put the cube down and walked away.
David’s tweet also lead to
I’ve never been a great one for points and the like in class, mostly due to my inability to be consistent enough in their use and unexamined distaste.