The Earth is flat and the Sun revolves around it: two theories that were assumed to be true long after the evidence to the contrary became overwhelming.
Richard Thaler, the author of Nudge, the popular work of social psychology, recently asked for other examples of "things we once thought were true and took forever to unlearn".

Sometimes it seems as though British politics exists simply to provide Thaler with case studies. Howard Flight, the MP who was forced out of the Commons by Michael Howard at the 2005 election for making a prediction deemed unhelpful to the Conservative cause, last week embarrassed the current Tory leader, who has just nominated him to return to Parliament as a member of the House of Lords.

The soon-to-be Lord Flight suggested that state benefits encourage claimants to have more children. "We're going to have a system where the middle classes are discouraged from breeding because it's jolly expensive, but for those on benefit there is every incentive," he told the Evening Standard. Both parts of that statement are demonstrably true, but the social psychology of groupthink requires everyone to perform their allotted roles in rituals as formalised as those of the Roman Catholic Church that condemned Galileo.

First, journalists report a "gaffe" – a word of almost theological definition, which is not used in normal English. Opposition politicians and commentators then condemn the maker of the gaffe, often for things that he or she has not said but for an implication or extrapolation. The third stage of the ritual involves disciplinary action and attempts to avoid it. In this case, Flight went through the full sequence of available responses, from "my words were taken out of context" (which they weren't) to an "unreserved apology" and a retraction. That proved enough to avoid stage four of the ritual, and to persuade David Cameron not to withdraw his nomination to the peerage.

Flight was held up to ridicule mainly because the word "breeding" recalled the speech by which Sir Keith Joseph destroyed his hope of leading the Conservative Party in 1974. That was when Sir Keith warned that "our human stock is threatened" by the "high and rising proportion of children" being born to adolescent mothers in "social classes four and five". What was offensive about that was the idea of eugenics – improving the race by selective breeding for strength, health or intelligence.

Flight said nothing of that kind. Yet the mere shadow of word-association diverts debate about tax and benefit policy into a gotcha ritual of heresy-hunting. He said that policy designed to alleviate child poverty has a perverse effect in encouraging people on benefits to have more children than they otherwise would.

This has been demonstrated by several academic surveys, not least a recent one by the unimpeachable Institute for Fiscal Studies. In December 2008 it published a paper entitled "Does welfare reform affect fertility?" It was barely reported in the press, for reasons in which social psychologists might be interested, because it found that, since Labour increased child-related benefits in 1999, "there was an increase in births (by around 15 per cent) among the group affected by the reforms". It is not eugenicist to point out that this can have undesirable consequences. Or, as Flight put it, rather mildly, "that's not very sensible".

A large part of the problem is that Tony Blair made a promise to "end child poverty" which was not deliverable but which could not be criticised because that would be to be "in favour of child poverty". It was, we can see now, a stupid pledge – one that Blair decided to put in his 1999 Beveridge lecture only 40 minutes before he delivered it, I am told. As a result, instead of focusing on the underlying causes of poverty, the government was driven to throw money at families with children in order to move them above an arbitrary definition of low income.

This is not the only example of the British media-politics nexus getting something very wrong for years, for high-minded motives. The crisis of the euro has proved what should have been obvious: that it was a bad idea to lock together currencies of very different economies. We were warned, by people often belittled as obsessive or xenophobic, that to bend the rules to let Italy join the euro was asking for trouble. That Greece's membership was a political gesture that would end in tears. That the peripheral countries would struggle to sustain their membership of the currency union if there were an economic crisis. And so it came to pass. Yet Blair wanted to stage a referendum on Britain's adoption of the euro as late as 2003 (not that he actually would have done, because there was no prospect of winning it). As far as I know, it is still formally Labour and Liberal Democrat policy that Britain should join.

Another issue is immigration, where any discussion of its possible adverse consequences is constrained by the prejudices of the liberal media. Yet when YouGov asked people to name Blair's greatest failure, rising immigration came top of the list, not the Iraq war.

It is, therefore, the richest of ironies that Howard Flight also said, in the interview in which he spoke the unspeakable truth: "We have reached the state of an elected tyranny of the professional politician. Partly because of the media scrutiny, MPs feel they cannot say anything except the blandest nonsense."

The question is: what other widely held assumptions will turn out to be wrong in decades to come? What evidence that contradicts the conventional view of the world is being ignored? What other political taboo subjects should be opened for debate? To get the thing started, I nominate "affordable housing". But there must be many more. Answers on a postcard, or its digital equivalent, to Richard Thaler at his Edge website.

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