Commenter “Tenn” asked me to estimate the IQ of several people, most notably Ronald Reagan, and what better day to do so than President’s Day?

In 2005, Reagan was elected the Greatest American of all time.  He was unbelievably influential because not only was he President for eight years, but his Vice President became President for four years, and his Vice President’s son became President for another eight years.  The Democratic party was so shell-shocked by Reagan’s success, that they moved economically to the right during the Clinton administration.

Reagan was influential, not only in changing the trajectory of American politics for decades, but was instrumental in making America the World’s sole super power.

Historians can debate whether his influence was positive or negative, but anyone who changed the World as much as Reagan did, would likely have an IQ way above 100.

On the other hand, as far as I know, Reagan was the only U.S. President to have been mentally impaired during old age.  Thus by definition, Reagan’s IQ during old age, was in the bottom 2.27% of American Presidents. This might be an underestimate because some U.S. Presidents have not (yet) lived to old age, on the other hand it might be an overestimate because Reagan might still have been the only one to become impaired, even if there had been 88 Presidents.  But assuming it’s roughly correct, it implies that if all 44 U.S. Presidents took an IQ test when they were very old, Reagan would have likely scored two standard deviations below the average U.S. President.

Of course, the relevant question is not how Reagan would have scored when he had Alzheimer’s, but how he would have scored at his peak.

In one of the most fascinating studies in the history of psychometrics, Ian Deary and a team of other scholars, tracked down 101 people who took an IQ test at age 11, and tested them again at age 77, on the exact same test!

They found a 0.63 correlation between IQ measured at 11 (by age 10,  IQ more or less stabilizes within measurement error) and IQ tested 66 years later.  Because the sample was somewhat restricted (the standard deviation was only 77%  of the national SD), the 0.63 correlation underestimates the relationship in a representative sample, however since the IQ variance of U.S. Presidents is similarly restricted (they seem to have a mean IQ of about 130  with an SD of 12, compared to the national white average of about 100 with an SD of about 15),  I will use the 0.63 correlation.

So if Reagan’s elderly IQ was two standard deviations below the elderly IQ of the average U.S. President, his peak IQ was likely 2 SD(0.63) = 1.26 SD below the peak IQ of the average U.S. President.  As mentioned above, U.S. Presidents have peak IQs (U.S. white norms) of about 130 with a restricted SD of 12.


Reagan’s likely peak IQ = -1.26(12) + 130

Reagan’s likely peak IQ = 115

The standard error of the estimate is 9.32, so you might say with 95% confidence that Reagan’s peak IQ was anywhere from 96 to 134.

Conservatives might argue he had a towering IQ of 134, since he was a supremely important President who wrote eloquent love letters to wife Nancy, but arrogant liberal professors might argue he was closer to 96 since in their eyes, he did enormous damage to the most vulnerable in society.

9.32 points might seem like a large standard error, but it’s actually smaller than the standard error you would get if you tried to predict someone’s Wechsler IQ from their SAT scores, or if you tried to predict someone’s Raven IQ from their Stanford Binet IQ.  Psychometrics is an inexact science.