Measuring IQ is an inexact science, which is why the Guinness Book of World Records eliminated the Highest IQ category in 1990.

Even with a test result in hand, the number can vary from exam to exam. Third party scoring or estimates based upon intellectual achievements are a popular way of getting a ballpark figure.

In other words, the following list is based on rumors and hearsay, but that's the only way to measure the world's smartest people.