Thomas Jefferson's IQ: Two Studies, Four Numbers, One Library

What two studies say

Jefferson appears in Catharine Cox's 1926 volume, The Early Mental Traits of Three Hundred Geniuses, among the cases rated 140 to 150 on childhood evidence. Her raters put him at 145 for birth to 17 and 150 for ages 17 to 26. Cox then adjusted both for the reliability of the underlying records, which produced 160 for each period.

Eighty years later the psychologist Dean Keith Simonton published "Presidential IQ, Openness, Intellectual Brilliance, and Leadership" in Political Psychology (vol. 27, no. 4, 2006). He estimated IQs for all 42 presidents from George Washington to George W. Bush. For the presidents Cox had already rated, Jefferson among them, he adopted her figures. For the rest he used a missing-data technique, expectation-maximization, to impute scores from ratings of intellectual brilliance and openness to experience.

Jefferson's four figures average 153.75. Press summaries round it to 154. His biography is at Thomas Jefferson; the method behind every such estimate is explained in historical IQ estimates, explained.

Why one source says 154 and another says 160

Both readings come from the same table. Writers who quote 160 cite the corrected young-adult column, the highest of the four. The historian Thomas C. Reeves did so when he reviewed the study for the History News Network in November 2006, listing Jefferson at 160 and John Quincy Adams at 175. Writers who quote 154 average all four columns, as Wikipedia's summary of the study does.

Neither figure is a measurement. The defensible statement is a range, roughly 145 to 160, centered in the mid-150s. For what those scores mean on a modern scale, see IQ 154 and IQ 160.

First on a second yardstick

Simonton's table also reports Intellectual Brilliance, a measure drawn from his earlier research on the presidents. Jefferson scores 3.1, the highest in the table. John Quincy Adams, whose IQ estimate is higher, scores 1.2.

The gap is instructive. Cox's ratings reward documented precocity before age 26. The brilliance measure reflects how the adult president struck observers and historians. Jefferson leads on the second and sits near the top on the first, which is why he is the usual answer to the question of America's most intellectual president.

The record behind the rating

Jefferson left the College of William & Mary in 1762, read law under George Wythe and was admitted to the bar in 1767. In June 1776, at 33, he drafted the Declaration of Independence. Notes on the State of Virginia appeared in French in 1785 and in English in 1787.

He designed Monticello, begun in 1768, and later the University of Virginia, chartered in 1819 and opened in 1825. He recommended a decimal system for American currency, designed a moldboard plow and devised a wheel cipher for encoding messages. In 1815 Congress bought his library, 6,487 catalogued volumes for $23,950, to replace the collection British troops had burned in the Capitol.

He wrote his own epitaph and listed three achievements: the Declaration, the Virginia Statute for Religious Freedom, enacted in 1786, and the University of Virginia. The presidency, 1801 to 1809, did not make the list.

The best-known tribute came from John F. Kennedy on April 29, 1962. Hosting Nobel laureates at the White House, he called the room the most extraordinary collection of talent ever gathered at the White House, "with the possible exception of when Thomas Jefferson dined alone."

Jefferson among presidents and founders

Two founders who never held the presidency also appear in Cox. Benjamin Franklin matches Jefferson's corrected 160. Alexander Hamilton, Jefferson's chief rival in Washington's cabinet, is rated 155. The achievement records are set side by side in Jefferson vs. Franklin.

What the number leaves out

An IQ estimate is silent on conduct. Jefferson wrote that all men are created equal and enslaved about 600 people over his lifetime. Cox's method was not built to weigh that contradiction, and no score can resolve it.

The estimates also compress a specific profile into one figure: a verbal and classifying mind that drafted law, catalogued a library and designed buildings. Simonton's own claim was narrower. Across 42 presidents, his estimated scores correlated with historians' evaluations of leadership performance. That is a statistical tendency across a group, not a verdict on any one man.

How we assessed this

Jefferson's four figures come directly from Table 1 of Simonton's 2006 paper and agree with his placement in Cox's table of contents. We checked two secondary accounts, Wikipedia's and Reeves's, to explain why published figures differ. Dates come from Britannica, Wikipedia, Monticello's research encyclopedia and the American Presidency Project's transcript of Kennedy's remarks.

Editorial standard: IQ figures for historical figures are retrospective estimates and are labeled as such; scores for living people appear only when the person disclosed them or a reputable outlet reported them. Corrections: contact the editors.