More than 50 percent of Harvard, Princeton, and MIT students can't answer this question correctly:
A bat and ball cost a dollar and ten cents. The bat costs a dollar more than the ball. How much does the ball cost? (Answer here.)
Why? Science says: In some ways, the smarter you are, the stupider you are. Or something. Jonah Lehrer reports at his New Yorker blog series Frontal Cortex (seriously, if you'd like to lose an afternoon, get on that thing):
West and colleagues weren’t simply interested in reconfirming the known biases of the human mind. Rather, they wanted to understand how these biases correlated with human intelligence. As a result, they interspersed their tests of bias with various cognitive measurements, including the S.A.T. and the Need for Cognition Scale, which measures “the tendency for an individual to engage in and enjoy thinking.”
The results were quite disturbing. For one thing, self-awareness was not particularly useful: as the scientists note, “people who were aware of their own biases were not better able to overcome them.” This finding wouldn’t surprise Kahneman, who admits in “Thinking, Fast and Slow” that his decades of groundbreaking research have failed to significantly improve his own mental performance. “My intuitive thinking is just as prone to overconfidence, extreme predictions, and the planning fallacy”—a tendency to underestimate how long it will take to complete a task—“as it was before I made a study of these issues,” he writes.