Immigration clearly ranks as one of the most important issues for voters in the lead up to the UK’s election. But public opinion doesn’t always match up with the evidence, and political parties can be led in different directions by both. With this in mind, the following takes stock of the different policies about immigration, as outlined in the parties’ manifestos.

Where do they stand?

UKIP makes the strongest claims about immigration causing harm. The Conservative and Labour manifestos also tend to emphasise the negatives of immigration, real or perceived.

Labour says the number of low-skilled immigrants is too high and points to no high-skilled categories where they’d welcome increase. Both the Conservatives and UKIP want migration to be lower overall. In their 2010 manifesto, the Conservatives proposed to reduce net migration to the tens of thousands – a “goal” which the coalition government dramatically missed. This time around, the party refers only to an “ambition”.

In contrast, the Liberal Democrats and the Green Party stress the benefits of immigration. Both parties propose more open policies, such as the restoration of post-study work visas for students and – in the case of the Greens – the abolition of minimum income requirements for the entry of spouses. The Greens are the most thoughtful on the global context, but their openness toward immigration is tempered by some scepticism toward immigration for business reasons or by the more affluent, citing worries about impacts on small businesses and house prices.

Among parties standing only in some parts of the UK, the SNP, Plaid Cymru and the Alliance Party are all fairly liberal, and concerned with the regional suitability of immigration criteria. Of the Northern Irish parties, the Ulster Unionist Party and the Democratic Unionist Party support immigration in moderation, while the nationalist Social Democratic and Labour Party and Sinn Féin are more or less silent on the matter.

An EU issue

UKIP’s approach is the most innovative and the most restrictive. The party wants to make sharp cuts to levels of immigration; an approach which is tied to their core proposal of withdrawing from the EU. Instead, the party wants to establish a points-based system which treats Commonwealth migrants comparably to European ones.

The Conservatives tie the issue of migration to a renegotiation of the terms of EU membership, as do the UUP. These parties question whether free movement meets the needs of established EU members. On the other hand, the SDLP’s positive comments on “free flow of people” within the EU is as close as the party comes to discussing immigration.

EU withdrawal is advocated also by several parties outside the mainstream whose manifestos talk at length about immigration. The Communist Party and Socialist Labour Party on the left, for example, regard the EU as an organisation promoting capitalist interests at workers’ expense. Both propose immigration policies outside the EU with humanitarian emphasis. The Socialist Labour Party propose a policy of zero net migration with priority for Commonwealth immigrants. For the English Democrats on the right, withdrawal not only from the EU but from a wide range of international agreements is regarded as essential to regaining full border control.

Boon, or burden on benefits?

For several parties, concerns about immigration from the EU focus on migrant benefit claims. Yet evidence suggests there is little reason to consider this a serious problem, and that restricting entitlements is unlikely therefore to discourage immigration.

The Conservatives, Labour, UKIP and DUP all want to delay receipt of benefits by migrants in various ways. The Conservative and Labour manifestos propose to rule out payment of child benefit for children abroad. This will raise issues with EU law, whether renegotiating terms of the UK’s EU membership or not.

Stresses on public services are a prominent theme in the Conservative, Labour and UKIP manifestos alike. The Conservatives propose a fund to alleviate such pressures, which bears similarities to a fund scrapped early in the last parliament.

The best evidence suggests that migrants pay taxes which more than cover the cost of benefits received, in cash or in kind. The net contribution of migrants should alleviate the cost of providing public services.

Where’s the evidence?

Costs imposed by migrants on the NHS are mentioned by the Conservatives, and are repeatedly emphasised in UKIP’s discussion of immigration. In fact, immigrants are typically healthier than natives on arrival, becoming more like them the longer they stay, and make similar use of health services.

Nonetheless, UKIP would require that most migrants arrive with private insurance. Unsurprisingly, the issue is also prominent for the National Health Action party. It proposes that stronger efforts be made to recover the costs of treating migrants, but opposes refusing treatment to anyone on ethical, economic and medical grounds.

Crime is another issue raised by Conservatives, Labour and UKIP in relation to immigration. In fact, evidence suggests migration is unassociated with changes in crime rates.

Housing also figures in some manifestos. The Conservative party worry about use of social housing, and UKIP about housing shortages. The Greens, on the other hand, worry about richer migrants pushing up house prices. Research on migration and housing is still developing, but evidence does not point to strong upward pressure on house prices.

Revival of student opportunity

UKIP alone discusses the burden which immigration imposes on schools. Such a burden might simply arise from growing numbers, or it might follow from the difficulty of educating children of mixed backgrounds together. The international evidence on the latter is ambiguous, but negative effects of high proportions of non-native speakers in the classroom on the performance of British-born children seems to be ruled out.

Instead, the treatment of foreign university students is the biggest issue linking education and migration. Liberal Democrats, Conservatives and Labour promise crackdowns on bogus institutions.

UKIP and the Lib Dems pledge to separate students in official statistics. Since the Lib Dems are not proposing to base targets on such statistics, the point of this is unclear. For UKIP, who do want to keep immigration down, but are not so averse to students, it makes more sense to exclude students from the count.

The most significant proposal here is reintroduction of the post-study work route, abolished under the current government, whereby students are permitted to work for two years after completing study. The Greens promise unconditional restoration, Plaid and the SNP propose restoration for students in Wales or Scotland, and the Lib Dems propose a reintroduction specifically for STEM (science, technology, engineering and mathematics) graduates.

STEM students have been shown to be particularly associated) with innovation, trade and entrepreneurship – issues which are largely absent from the discussion about migration in any manifesto.

Little effect on labour

A cap on skilled immigration would be retained by Conservative, Labour and UKIP. Indeed, UKIP would put a five-year moratorium on any unskilled immigration whatsoever, and restrict skilled immigration to 50,000 visas per year. By way of comparison, about 221,000 highly skilled non-student migrants are estimated to have entered the UK for work in the three years prior to 2013.

Evidence on how immigration affects average wages and employment finds no significant adverse effects. If there are labour market effects, then they hit workers on the lowest wages. The Conservatives, Labour, the Lib Dems and the Greens all propose crackdowns on exploitation, through new legislation or greater monitoring.

Questions of language proficiency appear in the manifestos of Conservatives, Labour, Lib Dems and Greens. The emphasis shifts from testing migrants’ English to offering English lessons, as the tone of the manifesto becomes more liberal.

Regional differences

To the extent that labour market restrictions are set nationally, they may be inappropriate for the demands of particular regions. Plaid, the SNP and the Alliance Party (in Northern Ireland) all call for greater regional sensitivity of policy in various respects, and complain about policies set to suit the south of England. But the unionist parties of Northern Ireland make no similar calls, and the nationalist parties of Northern Ireland say nothing on the issue.

One respect in which policy may be regionally discriminatory is nationally set income thresholds for family union, which may hurt families more in lower income regions. Some suggest such policies are intrinsically unjust. Plaid proposes a review; the Greens would drop the policy altogether. The Conservatives alone propose a toughening, while UKIP worries about sham marriages.

Full exit checks, frequently promised and already partly delivered, are proposed by Conservatives, Labour, Lib Dems and UKIP. Labour and UKIP both promise to expand border staff.

Protecting the persecuted

All main parties except the Conservatives reaffirm support for protecting victims of persecution. Labour, the Lib Dems and the Greens would end indefinite detention. The Lib Dems and the Greens go further, advocating allowing asylum seekers to work.

For those whose asylum claims fail, the Greens suggest a review of legal status, while the Lib Dems would abolish the Azure card system. The Greens want applicability of legal aid to immigration and asylum work extended.