There is an excellent new paper by Angela Lee Duckworth, et.al., and here is the punchline:
…material incentives in random-assignment studies increased IQ scores by an average of 0.64 SD, suggesting that test motivation can deviate substantially from maximal under low-stakes research conditions. The effect of incentives was moderated by IQ score: Incentives increased IQ scores by 0.96 SD among individuals with below-average IQs at baseline and by only 0.26 SD among individuals with above-average IQs at baseline.
Here is one popular summary of the results. I interpret the finding to suggest some mix of a) conscientiousness is more important than we think (when we think we are measuring the importance of IQ), and b) there are some smart people, smarter than we often think they are, and they pick and choose their spots.
For the pointer I thank Michelle Dawson.