I’ve wondered for a long time how IQ studies controlled for the motivation level of the participants. Turns out they don’t:

New work, led by Angela Lee Duckworth, a psychologist at the University of Pennsylvania, and reported online today in the Proceedings of the National Academy of Sciences explores the effect of motivation on how well people perform on IQ tests. While subjects taking such tests are usually instructed to try as hard as they can, previous research has shown that not everyone makes the maximum effort. A number of studies have found that subjects who are promised monetary rewards for doing well on IQ and other cognitive tests score significantly higher.

To further examine the role of motivation on both IQ test scores and the ability of IQ tests to predict life success, Duckworth and her team carried out two studies, both reported in today’s paper. First, they conducted a “meta-analysis” that combined the results of 46 previous studies of the effect of monetary incentives on IQ scores, representing a total of more than 2000 test-taking subjects. The financial rewards ranged from less than $1 to $10 or more. The team calculated a statistical parameter called Hedge’s g to indicate how big an effect the incentives had on IQ scores; g values of less than 0.2 are considered small, 0.5 are moderate, and 0.7 or higher are large.

Duckworth’s team found that the average effect was 0.64 (which is equivalent to nearly 10 points on the IQ scale of 100), and remained higher than 0.5 even when three studies with unusually high g values were thrown out. Moreover, the effect of financial rewards on IQ scores increased dramatically the higher the reward: Thus rewards higher than $10 produced g values of more than 1.6 (roughly equivalent to more than 20 IQ points), whereas rewards of less than $1 were only one-tenth as effective.

Full Story: What Does IQ Really Measure?

Full paper here.

(via Slate Star Codex)

Also of note: A study found that students who thought intelligence was malleable got better grades. “Convincing students that they could make themselves smarter by hard work led them to work harder and get higher grades. The intervention had the biggest effect for students who started out believing intelligence was genetic.” (paper).