Now is the time to slow down immigration What are we to make of the surging salience of immigration and the concomitant rise in support for Ukip? Are its supporters deluded, racist and misled, as the left would have it? Or are they voicing the wisdom of crowds, in angry alienation from metropolitan elites that have abandoned national identity, as claimed by the right? Public policy clearly needs to lower the temperature before popular discourse metastasises into something ugly. Evidently, it will need to address reasonable concerns and allay unwarranted fears. Instead, the political reaction has been predictably dysfunctional. The right smells a chance to castigate the left by exaggerating the costs of immigration: "we're swamped," and a theatrically aggressive message - "if its boat people getting swamped then it's not our problem." The left, terrified that any acknowledgment of costs would license hostility to migrants, clings to the narrative of the open door: that immigration delivers big economic gains. The key to getting out of this dangerous situation is to recognise that there is no inconsistency in asserting that past immigration has been modestly beneficial, while accepting that there is now a good case for curtailing further immigration. Existing immigrants are welcome, but future immigrants should be discouraged. Much of the fuss about migration has focused on its short-term economic impact: it is variously alleged to be crowding the low-skilled out of jobs (Ukip) or to be essential for growth or short-term fiscal receipts (the left and big business). In fact, the evidence is that these effects are minimal. A careful new study across Europe by Frédéric Doquier of the University of Louvain finds that the cumulative impact of a decade of immigration has changed wages by between 0% and 0.5%, depending on the country. The important effects of immigration are social and long term, not economic and short term. The key long-term social effects are probably on the overall size of the population and its diversity. As to population size, Britain is already one of the most crowded countries in Europe, and there is a sound environmental argument for protecting quality of life by discouraging further substantial increases. As to diversity, it involves a trade-off: as it increases, variety is enhanced but cohesion reduced. Variety is good but, unfortunately, as cohesion erodes voters become less willing to support generous welfare programmes. There is a universal psychological tendency for inconvenient truths to be denigrated, and this is certainly inconvenient for the left. But it is not speculation: I describe some of the supporting research in my book Exodus, and rigorous new experimental research by the Oxford political scientists Sergi Pardos and Jordi Muñoz finds that immigration has just this effect, especially on benefits that are targeted at the poor. This answers the question I started with. The trade-off between variety and cohesion affects social groups differently. The young, affluent middle classes are the big beneficiaries of variety. In contrast, those people on benefits, whether because they are unemployed or pensioners, are the most vulnerable to the weakening of cohesion. Ukip support is highly distinctive in that it is concentrated in areas of unemployment and retirement. While Ukip supporters may well be deluded, racist and misled, their opposition to further rapid immigration is indeed likely to be in their own best interest. There is a good case for confronting their delusions and racism, and countering the misleading drizzle of anti-immigrant anecdotes, but this would not make them accepting of continued high immigration. Might Ireland or Scotland provide models of more welcoming environments than England? During its boom, Ireland managed to accept larger proportionate numbers of immigrants than England with less opposition. Alex Salmond famously pitched that an independent Scotland would welcome immigration. Should a frigidly xenophobic England learn from Celtic warmth? Such denigration of the English is fatuous. The immigrants to Ireland were overwhelmingly Catholic east Europeans, and so integrated into the basic Irish unit of social organisation relatively easily. Further, the Irish welfare system remains rudimentary, so once the economy crashed many of its immigrants left. Whether this temporary surge in immigration was beneficial to Ireland is debatable: by facilitating the construction boom, it amplified the mega-bust that followed, but anyway it is not an option for England. As to Scotland, it already has zero net migration: immigrants overwhelmingly opt for England. A Labour attempt to disperse some Somali immigrants to Glasgow had to be abandoned because one was murdered. Salmond's pitch was merely cheap talk. There is no way of establishing whether further increases in diversity in England would be a net gain or a net cost. However, the rate at which migrants are assimilating appears to be slower than had been expected. Immigrants have tended to cluster, and this reduces social interaction outside the group. Hence, after the surge in immigration since 1997, it may be sensible to have a temporary phase of slower immigration while we take stock of its social consequences. The economic consequences of a pause would be negligible as long as students were exempted. It would be salutary for business to find that it had to train the existing workforce rather than poach trained workers from poorer countries: what is good for business is not necessarily good for the rest of us. Far from a pause licensing hostility to existing migrants, it may be necessary to avert hostility. While Ukip supporters should rationally fear continued rapid increases in diversity, they should welcome the integration of existing immigrants. The true focus of policy should be on what, realistically, can be done to implement a pause. For non-EU immigration we retain considerable policy freedom, but within the EU we have to get smart. We need to ask ourselves why so many would-be immigrants crowd at Calais: after all, their alternative to England is not their country of origin, but France and the entire open-access Schengen area, including prosperous Germany and generous Scandinavia. Perhaps it is that, unlike in the rest of Europe, access to our welfare system is not determined by past contributions. We could change that without EU permission. Or maybe it is that, owing to theatrical Tory opposition to identity cards, and the absence of a system of residence registers as exists in much of Europe, we are a paradise for illegality. We could change that too. Or perhaps it is that London is booming. If this is the real issue, then we have a reasonable basis for negotiation with the European commission. The right to impose temporary controls triggered by out-of-phase economic cycles, which could be invoked by any country that met the eligibility criteria, would not challenge the existential symbolism that many continental politicians attach to the principle of free movement. Ukip is on to the perfect issue: majority opposition to continued rapid immigration can be linked to the need to recover policy freedom from the European commission. The attempt to counter it with the message that continued immigration and EU membership are economically necessary is ineffective because it is seen by many ordinary people as a self-serving elite narrative that conceals contempt for their concerns. Breaking out of this is essential and feasible, but the left must take the lead. Two generations ago the US opened to China because Richard Nixon saw that only the right could free Americans from the myth of the "yellow peril." In Britain now, only the left can free us from the myth of the open door. Paul Collier is professor of economics and public policy at the Blavatnik School of Government at the University of Oxford, and the author of Exodus: Immigration and Multiculturalism in the 21st Century