How was it allowed to happen? How did politics in the US come to be dominated by people who make a virtue out of ignorance? Was it charity that has permitted mankind's closest living relative to spend two terms as president? How did Sarah Palin, Dan Quayle and other such gibbering numbskulls get to where they are? How could Republican rallies in 2008 be drowned out by screaming ignoramuses insisting that Barack Obama was a Muslim and a terrorist?

Like most people on my side of the Atlantic, I have for many years been mystified by American politics. The US has the world's best universities and attracts the world's finest minds. It dominates discoveries in science and medicine. Its wealth and power depend on the application of knowledge. Yet, uniquely among the developed nations (with the possible exception of Australia), learning is a grave political disadvantage.

There have been exceptions over the past century - Franklin Roosevelt, JF Kennedy and Bill Clinton tempered their intellectualism with the common touch and survived - but Adlai Stevenson, Al Gore and John Kerry were successfully tarred by their opponents as members of a cerebral elite (as if this were not a qualification for the presidency). Perhaps the defining moment in the collapse of intelligent politics was Ronald Reagan's response to Jimmy Carter during the 1980 presidential debate. Carter - stumbling a little, using long words - carefully enumerated the benefits of national health insurance. Reagan smiled and said: "There you go again." His own health programme would have appalled most Americans, had he explained it as carefully as Carter had done, but he had found a formula for avoiding tough political issues and making his opponents look like wonks.

It wasn't always like this. The founding fathers of the republic - Benjamin Franklin, Thomas Jefferson, James Madison, John Adams, Alexander Hamilton and others - were among the greatest thinkers of their age. They felt no need to make a secret of it. How did the project they launched degenerate into George W Bush and Sarah Palin?

On one level, this is easy to answer. Ignorant politicians are elected by ignorant people. US education, like the US health system, is notorious for its failures. In the most powerful nation on earth, one adult in five believes the sun revolves round the earth; only 26% accept that evolution takes place by means of natural selection; two-thirds of young adults are unable to find Iraq on a map; two-thirds of US voters cannot name the three branches of government; the maths skills of 15-year-olds in the US are ranked 24th out of the 29 countries of the OECD. But this merely extends the mystery: how did so many US citizens become so stupid, and so suspicious of intelligence? Susan Jacoby's book The Age of American Unreason provides the fullest explanation I have read so far. She shows that the degradation of US politics results from a series of interlocking tragedies.

One theme is both familiar and clear: religion - in particular fundamentalist religion - makes you stupid. The US is the only rich country in which Christian fundamentalism is vast and growing.

Jacoby shows that there was once a certain logic to its anti-rationalism. During the first few decades after the publication of The Origin of Species, for instance, Americans had good reason to reject the theory of natural selection and to treat public intellectuals with suspicion. From the beginning, Darwin's theory was mixed up in the US with the brutal philosophy - now known as social Darwinism - of the British writer Herbert Spencer. Spencer's doctrine, promoted in the popular press with the help of funding from Andrew Carnegie, John D Rockefeller and Thomas Edison, suggested that millionaires stood at the top of a scala natura established by evolution. By preventing unfit people being weeded out, government intervention weakened the nation. Gross economic inequalities were both justifiable and necessary.

Darwinism, in other words, became indistinguishable from the most bestial form of laissez-faire economics. Many Christians responded with revulsion. It is profoundly ironic that the doctrine rejected a century ago by such prominent fundamentalists as William Jennings Bryan is now central to the economic thinking of the Christian right. Modern fundamentalists reject the science of Darwinian evolution and accept the pseudoscience of social Darwinism.

But there were other, more powerful, reasons for the intellectual isolation of the fundamentalists. The US is peculiar in devolving the control of education to local authorities. Teaching in the southern states was dominated by the views of an ignorant aristocracy of planters, and a great educational gulf opened up. "In the south", Jacoby writes, "what can only be described as an intellectual blockade was imposed in order to keep out any ideas that might threaten the social order."

The Southern Baptist Convention, now the biggest Protestant denomination in the US, was to slavery and segregation what the Dutch Reformed Church was to apartheid in South Africa. It has done more than any other force to keep the south stupid. In the 1960s it tried to stave off desegregation by establishing a system of private Christian schools and universities. A student can now progress from kindergarten to a higher degree without any exposure to secular teaching. Southern Baptist beliefs pass intact through the public school system as well. A survey by researchers at the University of Texas in 1998 found that one in four of the state's state school biology teachers believed humans and dinosaurs lived on earth at the same time.

This tragedy has been assisted by the American fetishisation of self-education. Though he greatly regretted his lack of formal teaching, Abraham Lincoln's career is repeatedly cited as evidence that good education, provided by the state, is unnecessary: all that is required to succeed is determination and rugged individualism. This might have served people well when genuine self-education movements, like the one built around the Little Blue Books in the first half of the 20th century, were in vogue. In the age of infotainment, it is a recipe for confusion.

Besides fundamentalist religion, perhaps the most potent reason intellectuals struggle in elections is that intellectualism has been equated with subversion. The brief flirtation of some thinkers with communism a long time ago has been used to create an impression in the public mind that all intellectuals are communists. Almost every day men such as Rush Limbaugh and Bill O'Reilly rage against the "liberal elites" destroying America.

The spectre of pointy-headed alien subversives was crucial to the election of Reagan and Bush. A genuine intellectual elite - like the neocons (some of them former communists) surrounding Bush - has managed to pitch the political conflict as a battle between ordinary Americans and an over-educated pinko establishment. Any attempt to challenge the ideas of the rightwing elite has been successfully branded as elitism.

Obama has a lot to offer the US, but none of this will stop if he wins. Until the great failures of the US education system are reversed or religious fundamentalism withers, there will be political opportunities for people, like Bush and Palin, who flaunt their ignorance.



monbiot.com

• This article was amended on Tuesday November 4 2008. The Southern Baptist Convention is not the biggest denomination in the US. We meant to describe it as the biggest Protestant denomination. This has been corrected.