A similar approach is used when gauging racial attitudes in Britain, where the likelihood that people will lie about their prejudice is also strong. The British Social Attitudes survey and Professor Paul Sniderman in the US developed a technique of asking people both "How you yourself" feel about people of other races and then "How people in your street" feel about them to get a more honest answer. Although it is possible for individuals to answer truthfully that they are not prejudiced but their neighbours are, if everybody says it, then someone must be fibbing.
Sure enough, when given the option of delegating prejudice to their neighbours rather than taking responsibility for it themselves, the number of those who are "very prejudiced" doubles.
When it comes to debates on race and immigration, honesty is usually the first casualty, recent contributions by David Goodhart on these pages and elsewhere being no exception. "We are the sum of the things we pretend to be," said the American writer Kurt Vonnegut. "So we must be careful what we pretend to be."
Mr Goodhart pretends to be a liberal intellectual daring enough to tackle what he calls "the progressive dilemma" of balancing solidarity and diversity. Whatever we think of his arguments, we should take him at his word and test his thesis against the standards he has set himself. If he is liberal we should reasonably assume that he is anti-racist. If he is an intellectual we can presume his thesis will stand up to basic scrutiny.
On the first count he fails miserably. Throughout his essay he wilfully confuses issues of race and immigration as though they were inevitably linked and inherently interchangeable. What starts as a thesis about managing migration to preserve the welfare state - the fact that the NHS and many other public services owe their existence to mass migration earns an entire parenthesis towards the end - develops into a diatribe about the flaws of ethnic diversity.
Take the following: "Immigration and asylum: about 9% of British residents are now from ethnic minorities, rising to almost one-third in London." Given that more than half of non-white people were born in Britain, what does this fact have to do with immigration or asylum?
His failure to distinguish between race (the colour of people) and place (the movement of people) leads him up a predictable cul-de-sac.
"We need to be reassured that strangers, especially those from other countries, have the same idea of reciprocity as we do," he argues, prompting some obvious questions. Who does he consider to be the strangers from our own country? How did they become strangers in their own land? And who does he mean by "we"?
His articles are littered with assumptions about "us" and "them" and peppered with references to a "common culture" and "homogeneity", as though such terms are not only universally agreed but eternally static. No wonder he concludes that "National citizenship is inherently exclusionary". Of course it is, if defined by race and frozen in time.
There can be no liberal understanding of either race or immigration that fails to grasp the basic premise that British does not mean white and black does not mean foreign. The closest Goodhart gets is referring to "Britain's established minorities with whom we are willing to share ..." In the face of such liberal generosity one yearns for nationalist candour. Goodhart does not throw down a gauntlet to the left but rolls out the red carpet for the right.
Rip the figleaf of race from Goodhart's thesis and the central flaw is laid bare. The issue at the heart of modern-day immigration is not ethnicity but economics. Rather than ask why billions living on less than $1 a day see Britain as a paradise and what can be done about it, he simply wraps himself in the flag - a comfort blanket for all seasons.
"The national community remains the basic unit of human political organisation and will remain so long into the future," he says.
This is true, as far as it goes. But that isn't very far unless you also acknowledge that globalisation remains the basic thrust of economic development and will remain so long into the future.
In a world that has placed a huge premium on the free movement of capital, we should not be surprised if what follows is a desire for the free movement of labour. As free trade and deregulation force privatisation and low tariffs on poor countries so residents of those countries will move to where the wealth is concentrated. We used to slam the eastern bloc for not letting people out; now we won't let them in.
Herein lies the hypocrisy of the desire of Europe's wealthier nations to discourage immigration from accession countries. With no commensurate attempt to regulate the movement of capital to and from those countries, they allow the rich to go in search of profit while denying the poor the right to seek work.
People come to the west looking for opportunity because opportunism, in the form of western capital, has gone to the developing world looking for them. If we want to manage migration, we should start by looking at fair trade and international aid. Building higher walls and slashing the welfare rights of migrants may offer temporary respite from the chaos we wreak beyond our borders. But it will do little to relieve the source of desperation that forces them to leave.
There can be no meaningful comprehension of migration to the west, let alone a liberal response to it, that does not take account of poverty in the south.
But Goodhart is right on two counts. First, we do need a sensible and open discussion about race, ethnicity and citizenship in this country: one that takes into account our past as an empire (not just the soldiers who fought for it, but the civilians who were massacred by it); our present as a devolved monarchy within the EU; and our future as a multiracial nation in a global economy that seeks to preserve a welfare state. Sadly he answers none of these questions, offering instead dubious assertions: "To put it bluntly - most of us prefer our own kind." That smacks of bitter, white whine in new bottles.
Second, there is indeed a progressive dilemma. It's the dilemma of what to do with people who pose as progressives and preach like reactionaries.
