In 2006, University of California at Davis psychology professor Dean Simonton completed a comprehensive study examining the "intellectual brilliance" of 42 US presidents.
The top 15 who appear on this list were compiled by Libb Thims — an American engineer who compiles high IQ scores as a hobby — using the results of Simonton's study.
Because IQ scores weren't available for all of the presidents, Simonton estimated their scores based on certain personality traits noted in their biographies that would indicate a higher-than-average level of intelligence, such as "wise," "inventive," "artistic," "curious," sophisticated," "complicated," and "insightful."
Simonton then gave each president a score based on his personality traits, which he then interpreted as a measure of the chief executives' "Intellectual Brilliance."
See the top 15 smartest presidents in the gallery below: