### The smarter you are, the more likely you will get some simple questions wrong.

**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?**

The vast majority of people respond quickly and confidently, insisting the ball costs ten cents.

Think again.

When people face an uncertain situation, they don’t carefully evaluate the information or look up relevant statistics. Instead, their decisions depend on a long list of mental shortcuts, which often lead them to make foolish decisions.

These shortcuts aren’t a faster way of doing the math; they’re a way of skipping the math altogether. Asked about the bat and the ball, we forget our arithmetic lessons and instead default to the answer that requires the least mental effort.

**In a lake, there is a patch of lily pads. Every day, the patch doubles in size. If it takes 48 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake?**

Your first response is probably to take a shortcut, and to divide the final answer by half. That leads you to twenty-four days. But that’s wrong. The correct solution is forty-seven days.

Perhaps our most dangerous bias is that we naturally assume that everyone else is more susceptible to thinking errors, a tendency known as the *bias blind spot*.

Sadly, intelligence seems to make things worse. More cognitively sophisticated participants in numerous studies by Daniel Kahneman showed larger bias blind spots. Smarter people (at least as measured by SAT scores) and those more likely to engage in deliberation were even more vulnerable to common mental mistakes.

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