Hey folks, this is a really excellent discussion of the issues and research around using extrinsic rewards as a way to motivate behavior. Chris Hecker is looking at the question through the lens of game design, but it really, really applies to learning design as well. 
There’s a write-up at the website, and a recording of the talk if you scroll down. It’s long-ish, but well worth the listen.
Found this via Amy Jo Kim on twitter: https://twitter.com/amyjokim