Self-deprecating, too liberal for their own good, today's progressives stand back and watch, hands over their mouths, as the social vivisectionists of the right slice up a living society to see if its component parts can survive in isolation. Tied up in knots of reticence and self-doubt, they will not shout stop. Doing so requires an act of interruption, of presumption, for which they no longer possess a vocabulary.

Perhaps it is in the same spirit of liberal constipation that, with the exception of Charlie Brooker, we have been too polite to mention the Canadian study published last month in the journal Psychological Science, which revealed that people with conservative beliefs are likely to be of low intelligence. Paradoxically it was the Daily Mail that brought it to the attention of British readers last week. It feels crude, illiberal to point out that the other side is, on average, more stupid than our own. But this, the study suggests, is not unfounded generalisation but empirical fact.

It is by no means the first such paper. There is plenty of research showing that low general intelligence in childhood predicts greater prejudice towards people of different ethnicity or sexuality in adulthood. Open-mindedness, flexibility, trust in other people: all these require certain cognitive abilities. Understanding and accepting others – particularly "different" others – requires an enhanced capacity for abstract thinking.

But, drawing on a sample size of several thousand, correcting for both education and socioeconomic status, the new study looks embarrassingly robust. Importantly, it shows that prejudice tends not to arise directly from low intelligence but from the conservative ideologies to which people of low intelligence are drawn. Conservative ideology is the "critical pathway" from low intelligence to racism. Those with low cognitive abilities are attracted to "rightwing ideologies that promote coherence and order" and "emphasise the maintenance of the status quo". Even for someone not yet renowned for liberal reticence, this feels hard to write.

This is not to suggest that all conservatives are stupid. There are some very clever people in government, advising politicians, running thinktanks and writing for newspapers, who have acquired power and influence by promoting rightwing ideologies.

But what we now see among their parties – however intelligent their guiding spirits may be – is the abandonment of any pretence of high-minded conservatism. On both sides of the Atlantic, conservative strategists have discovered that there is no pool so shallow that several million people won't drown in it. Whether they are promoting the idea that Barack Obama was not born in the US, that man-made climate change is an eco-fascist-communist-anarchist conspiracy, or that the deficit results from the greed of the poor, they now appeal to the basest, stupidest impulses, and find that it does them no harm in the polls.

Don't take my word for it. Listen to what two former Republican ideologues, David Frum and Mike Lofgren, have been saying. Frum warns that "conservatives have built a whole alternative knowledge system, with its own facts, its own history, its own laws of economics". The result is a "shift to ever more extreme, ever more fantasy-based ideology" which has "ominous real-world consequences for American society".

Lofgren complains that "the crackpot outliers of two decades ago have become the vital centre today". The Republican party, with its "prevailing anti-intellectualism and hostility to science" is appealing to what he calls the "low-information voter", or the "misinformation voter". While most office holders probably don't believe the "reactionary and paranoid claptrap" they peddle, "they cynically feed the worst instincts of their fearful and angry low-information political base".

The madness hasn't gone as far in the UK, but the effects of the Conservative appeal to stupidity are making themselves felt. This week the Guardian reported that recipients of disability benefits, scapegoated by the government as scroungers, blamed for the deficit, now find themselves subject to a new level of hostility and threats from other people.

These are the perfect conditions for a billionaires' feeding frenzy. Any party elected by misinformed, suggestible voters becomes a vehicle for undisclosed interests. A tax break for the 1% is dressed up as freedom for the 99%. The regulation that prevents big banks and corporations exploiting us becomes an assault on the working man and woman. Those of us who discuss man-made climate change are cast as elitists by people who happily embrace the claims of Lord Monckton, Lord Lawson or thinktanks funded by ExxonMobil or the Koch brothers: now the authentic voices of the working class.

But when I survey this wreckage I wonder who the real idiots are. Confronted with mass discontent, the once-progressive major parties, as Thomas Frank laments in his latest book Pity the Billionaire, triangulate and accommodate, hesitate and prevaricate, muzzled by what he calls "terminal niceness". They fail to produce a coherent analysis of what has gone wrong and why, or to make an uncluttered case for social justice, redistribution and regulation. The conceptual stupidities of conservatism are matched by the strategic stupidities of liberalism.

Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics on both sides of the Atlantic continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down.

Twitter: @georgemonbiot