"The median Turker...had completed 300 total academic studies"

Workers on Mechanical Turk are experienced participants, which may be a problem for some studies.

“If you’re running social psychology studies on Turk, watch out, because [the subjects] have gotten experienced, and that can change effects,” Rand said. “So if you run my experiment on Turk right now, you won’t get any effect. Which sucks for me.”

To be clear, extreme experience isn’t always a problem, Rand said. There are some psychological tests that are so robust that no amount of experience will override the effect, said Jesse Chandler, a researcher at the University of Michigan’s Institute for Social Research. The Stroop effect, for example, which involves identifying colors when the color of a word doesn’t match the color spelled out by the text. When the word “red,” for example, is colored green, it takes longer to override the automatic reading of the text and choose green.