The left is playing a dangerous game in ignoring the evidence over immigration

In the late 1950s, large numbers of Chinese people became convinced sparrows were devouring grain seed and causing devastating famines.

All over China farmers and city folk began to persecute sparrows. They raided nests, killing chicks. They banged drums and cymbals to frighten the birds whenever they tried to land. The creatures eventually dropped out of the sky from exhaustion. The species was more or less wiped out in China.

But the anti-sparrow campaign was based on a lie.

The birds were not the cause of China’s crop failures. In fact, the slaughter of the birds contributed to an insect blight which made the famine still worse.

The great Chinese sparrow persecution is a reminder that a population can passionately believe something that simply isn’t true.

So how should the Chinese authorities have reacted to that popular conviction that sparrows were the cause of their misery? Should they have firmly told the people that there was no scientific evidence to support the theory that sparrows were causing famine and urged them to cease the irrational persecution?

Or would that have been patronising and counterproductive? Should the widespread belief have been humoured? Should the Chinese have been told that, yes, sparrows were indeed a menace and that it made sense to reduce their numbers?

That may sound ridiculous. Yet there is a growing branch of liberal opinion which suggests British politicians, pundits and intellectuals should let people assert things which are not true.

The academic evidence we have is very clear that immigration does not undermine average UK living standards, but actually enhances them. Some researchers have found that there is a negative impact on the wages of unskilled natives – but only a mild one. Overall the impact is positive.

Yet some on the liberal left, despite acknowledging this evidence, are moving to the view that telling people that they’re wrong when they complain of a negative economic impact of immigration is condescending.

To outline what the academic research shows on migration “comes across as either an accusation of stupidity or a symptom of elite arrogance”, as one newspaper commentator puts it.

“Telling someone ‘you’re wrong, here are the facts’ only alienates them further” the think tank British Future has argued. The suggestion is that there should be less stress on what the evidence shows and much more on engaging people in a conversation.

The critique misdiagnoses the illness. The dominant reason people believe myths about the negative impact of immigration is not because they dislike being talked down to by arrogant metropolitan liberals, but because they’re incessantly fed those beguiling stories by the right-wing media and they are not properly rebutted by mainstream politicians.

And why? In a column last week my colleague John Rentoul quoted the old Soviet leader Leonid Brezhnev (as recalled by Richard Nixon): “If the people see an imaginary river out there, you don’t tell them there’s no river there, you build an imaginary bridge over the imaginary river. That would be a politically successful strategy.”

But while imaginary bridge building can sometimes deliver short -term relief for politicians, in the long term it’s often destructive.

Consider David Cameron’s infamous pledge to reduce net migration rates to the “tens of thousands” to assuage public anxiety on the subject. This crude target was always unlikely to be hit and the policy failure ended up reinforcing public anger over immigration that helped deliver the Brexit result.

Some will find the comparison between the Chinese sparrow persecution and the anti-immigration sentiment in Britain today flawed, perhaps even offensive. The Chinese authorities were never going to tell the truth about sparrows because they were the ones misinforming people.

Mao Zedong orchestrated the campaign to distract from his other catastrophic mistakes in the Great Leap Forward.

And 21st century Britain is a democracy, not a totalitarian state. People can make up their own minds can't they?

Yet the differences are not as great as we’d like to think. People have been sold a simple but incorrect story that their economic grievances stem from an influx of foreigners. And despite clear evidence that curbing migration will actually damage our prosperity and make people worse off, Theresa May is now preparing to set government policy as if this story is substantially true.

One of the barriers to a country like Britain descending into the totalitarian abyss of China in the 1950s is that we have political leaders and a media (or at least part of it) that tell the truth and a culture of respect for scientific evidence. To diminish the truth-telling diminishes the barrier.

Democracies are not, sadly, inherently immune to the virus of ignorant and dead-end populism. Look at what is happening across the Atlantic where Donald Trump’s lies and conspiracy theories have taken hold of the minds of a distressingly large number of Americans.

It’s true that politicians should not label all people who voice concerns about immigration as either racist or stupid. It’s also sensible to address the underlying anxieties that many people have when they talk about immigration, such as stagnant wages, insecurity at work, pressure on public services and a shortage of housing. Localised infrastructure stresses and community integration issues should, of course, be taken seriously.

But to downplay scientific evidence, or to tacitly admit that there is some element of truth in the myths that immigrants make natives economically worse off is folly and will ultimately do more harm than good.

The central problem with the national debate over immigration in the UK is not that the truth is off-putting but that it simply isn’t heard enough. And liberals should never be reticent or ashamed about telling it as it is.