Estimated IQ scores of US presidents, each of whom was in power at one time, were published in 2006 in the journal Political Psychology based on research by University of California professor Dean Simonton. So the most intelligent president of the United States was John Adams. His IQ is 173. Second on the list of the smartest presidents was Thomas Jefferson, with an IQ of 160. The 35th president John Fitzgerald Kennedy, who did not stay in office for 3 years after he was assassinated in 1963, is considered the third among the smartest politicians. His IQ was determined at 158. The fourth most intelligent among the presidents was Bill Clinton, his IQ was estimated at 156. The five most intelligent presidents are closed by James Madison, one of the fathers of the Constitution, who had an IQ of 155. But George W. Bush has the lowest IQ, he was estimated at 91 points.