He “is definitely intelligent . . . certainly smart enough to be president of the United States”, says Dean Keith Simonton, a psychologist at the University of California.

But his intellect falls below all other presidents of the past 110 years except Warren Harding, who was in the White House briefly in the 1920s and regarded as a failed president.

Bush’s estimated IQ is about 20 points below that of his predecessor, Bill Clinton, “a disparity that may have created a contrast effect that made any intellectual weaknesses all the more salient”.

Simonton has drawn up a table of estimated presidential intelligence by amassing data created by other researchers. Writing in the journal Political Psychology he says that estimates of Bush’s IQ range between 111.1 and 138.5, with a mean of 120, “which is about the average for a college graduate in the United States”.

Clinton’s IQ ranged between 135.6 and 159, and Ronald Reagan’s between 118 and 141.9. John Quincy Adams, president from 1825-9, was the cleverest with a range of 165 to 175, well into genius territory.

The data used by Simonton was created by the filtering and analysis of personality descriptions from biographical sources — an academically recognised system known as a “historiometric” study.

Bush may be “much smarter” than the findings imply, says Simonton, but he scores particularly unimpressively for “openness to experience, a cognitive proclivity that encompasses unusual receptiveness to fantasy, aesthetics, actions, ideas and values. In the general population this factor is positively associated with intelligence”.

Bush’s openness score of zero — compared with 82 for Clinton and John F Kennedy, 95 for Abraham Lincoln and 99.1 for Thomas Jefferson — “placed him at the very bottom of US presidents”.

This assessment can only be considered tentative because of lack of available evidence on a sitting president, but it is corroborated by a measure of Bush’s “integrative complexity”. Simonton says: “Low scorers on integrative complexity can only see things from a single perspective — their own.”

Bush’s score, he says, is comparable to “extremist Islamic fundamentalists in the Taliban and Al-Qaeda leadership — with the notable exception of Osama Bin Laden, who is lower still”.

Trevor McCrisken, lecturer in American politics at Warwick University, said: “This is going to give added ammunition to those who dislike Bush, and who particularly dislike his folksiness, and his apparent lack of intellectual vigour. A major part of his public persona, to some extent, I think deliberately, is that he is not an intellectual. But he went to Yale, he has had an exclusive upbringing and he is by no means a dimwit.”

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