The Conservatives' pledge to reduce net migration "from the hundreds of thousands to the tens of thousands" has been criticised for damaging economic interests and foreign relations and undermining family and refugee rights. As successive figures have shown net migration rise, it seems increasingly clear that the Government cannot meet the target, exposing a significant gap between political rhetoric and the reality of UK immigration control.

All liberal democratic countries that attract immigrants face a similar tension: a gap between what governments say to appease public opinion and what can feasibly be delivered in practice. While public opinion tends to be largely pro-restriction, a range of pressures dictates a more liberal approach in practice: economic interests and business needs; foreign and trade relations; EU free movement and human rights commitments; and concerns about inter-ethnic relations. These all constrain the capacity of liberal governments to introduce restrictive policies.

Liberal democratic countries have responded to this gap by adopting three main approaches. The first is an elitist style of policy making. Mainstream political parties agree to keep immigration off the agenda. Decision-making is largely driven by political elites and business leaders, ensuring there is sufficient foreign labour to fill gaps and underpin productivity and growth.

This elitist approach characterised the immigration policies of many Western European countries in the 1950s and 1960s. France, Germany, the Netherlands and the UK all experienced large-scale immigration, largely in the absence of party political debate.

Since the 1980s, however, immigration has become politicised in most European countries, foreclosing the possibilities for policy making behind closed doors. Nor is such an approach desirable, given the conflict with principles of democratic representation and accountability.

The second option has been to de-couple rhetoric and practice. Governments have combined largely symbolic measures to crack down on irregular migration, with toleration of sizeable numbers of undocumented residents in practice, often recruiting labour migrants through a series of obscure administrative loopholes despite having committed themselves to limiting immigration.

This approach is, again, hardly desirable, given the lack of accountability. It is also not feasible for the UK Government. By setting a target, it has put in place a robust system for monitoring the effectiveness of its policies. This constrains its ability to fudge outcomes.

So we come to the third option: launching a more honest and open debate about the limits to immigration control. This implies coming clean about the constraints facing liberal democracies, and about the ways in which immigration benefits our economy and society.

Germany is a good example of this approach. Its cross-party immigration commission of 2001 brought together representatives of the main political parties, employers and trade unions, church groups and non-government organisations. It drew on a wide range of evidence to produce balanced analysis and recommendations on immigration, integration and refugee policy. The commission generated a discernible shift in political debate, paving the way for measures to make German immigration policy more responsive to economic and demographic needs, address integration problems, and respond more generously to the refugee crisis.

The prospects for this type of cross-party debate appear remote at the UK level. The Conservative Government doesn’t seem anywhere close to renouncing its restrictive rhetoric, especially with the prospect of difficult discussions around European free movement in the run-up to the EU referendum.

But political dynamics in Scotland are more favourable. All parties have shown themselves open to a more constructive debate on Scotland's immigration needs. Scottish business and higher education have been outspoken in their opposition to the net migration target. Also, opinion polls have suggested that the public is relatively less anti-immigration than the rest of the UK (although it is still largely resistant).

A cross-party commission to debate immigration in Scotland needn't ignore or suppress public concerns. It would need to encourage open discussion, respecting people's anxieties about the impacts of immigration. But it could help expose some of the misconceptions about immigration and foster a more realistic debate about the opportunities and constraints in immigration policy. Who knows, it might even set the tone for a more progressive debate south of the Border. That has to be better than the present debate which revolves around a damaging and discredited net migration target.

Christina Boswell is professor of politics at the University of Edinburgh. This article is based on research sponsored by the Economic and Social Research Council.