THE Conservative Party promised ahead of its election victory in 2010 that it would bring annual net migration below 100,000 a year. As the economy has grown, sucking in foreign workers, the government has conspicuously failed to meet this goal: net migration in the year to June 2015 was 336,000, a record. However, one small but socially significant subsection has declined and remained low: immigration by Britons’ foreign spouses.

In 2012 the government introduced a new requirement that British citizens and permanent residents meet an income threshold before being allowed to bring in a partner from outside the European Union. The threshold is £18,600 ($26,500), or higher if children are to come too. This gives Britain the strictest policy on family unification of 38 rich countries, according to the Migration Policy Group, an NGO. The rules have been challenged in a case that will be heard by the Supreme Court next month.

The income threshold is high: the Migration Observatory at Oxford University calculates that 41% of British citizens would not meet it. And its reach is uneven: whereas almost three-quarters of men clear it, most women do not. In poor parts of the country, such as the North East, twice as many are ruled out as in the wealthy capital. And in London white people are twice as likely to be able to import a partner as non-whites. So far the new rules are reckoned to have affected up to 15,000 children, many kept apart from a parent.

The government projected in 2012 that 13,600-17,800 people per year would be prevented from coming to Britain as a result of the changes. Though the true figure is unknowable, the number of spousal visas granted fell by nearly one-third following the rule’s introduction (see chart). Those most affected are Pakistanis, who account for almost one-fifth of such visas. The government has not trumpeted it, but the new policy has an additional motive. Some believe that the ability to bring in spouses from overseas has put a brake on integration, especially among Pakistanis. About three-quarters of spousal visas are issued to women. David Goodhart of Policy Exchange, a think-tank, says that the constant replenishment of first-generation wives has been a bar to integration with British society. Saira Grant of the Joint Council for the Welfare of Immigrants, which is supporting the appellants at the Supreme Court, argues that “integration is not achieved by an artificial financial threshold”. She says there is no evidence that migrant spouses are a burden to the state. Yet studies suggest family migrants do have lower employment rates than the British average, though more than half work after their arrival. A quirk of the income threshold is that it does not apply to citizens of other EU countries, meaning they can bring their non-EU spouses to Britain without satisfying any income requirement. Some Britons get around the rules by moving to a European country where they meet up with their spouse and, after living there for a short period, returning together to Britain under the EU’s free-movement laws.

One of the issues the Supreme Court may examine is whether the foreign spouse’s income might count towards the threshold. At present, some low-earning British expatriates have trouble moving back from abroad with their foreign spouse, even if that partner is a high earner. The court has the power to declare these rules unlawful, though that is unlikely, believes Ms Grant. In the absence of many other ways of reducing immigration, the government will want to cling on to the strict new rules, for better or worse.