Ever since David Cameron pledged to significantly reduce net migration back in 2010, the term has featured prominently within British political and media debates. The Vote Leave campaign made net migration a central theme in the 2016 referendum on Britain’s EU membership, and migration continues to be one of the most contentious issues in British politics.

In basic terms, net migration is the difference between the number of people arriving to and leaving from an area within a given period of time. On a national scale, it is the balance of immigration and emigration. So if the net migration value is positive it means more people have immigrated to a country than emigrated. Conversely, if the value is negative, more people would have emigrated than immigrated.

The latest figures for the UK estimate that net migration in 2015 was +327,000, with roughly twice as many people arriving from abroad than leaving.

The current government’s policy is to reduce net migration below 100,000. But the Office for Budget Responsibility projected in March 2016 that the net migration rate will be 185,000 in 2021. The UK is the only country in Europe with an official maximum net migration target.

Net migration in the UK is monitored by the Office of National Statistics (ONS), which uses the international standard designation of long-term migrant (LTM) to classify immigrants and emigrants in the UK. LTM’s are defined as people who resettle in a new country for a period of at least a year. The number of LTM’s arriving and leaving Britain is calculated through the International Passenger Survey, which has been used since 1961 to collect data from travellers arriving and leaving the UK.

The ONS crosschecks its net migration figures with data from the national census. After the last census in 2011, the ONS published adjusted figures for the period 2001 to 2011 that showed overall net migration in that decade was significantly higher than its statistics had initially suggested. For example, in 2008 the ONS estimated net migration at 163,000, while the revised figure using the 2011 census data estimated it was actually 229,000.

Long-term international migration to the UK, 2006 to 2016. Office for National Statistics

How Britain compares

On a global scale, 71 countries will have positive net migration in 2016, according to data collated by the CIA in the US. This includes economically advanced counties such as the US, Australia, Germany and the UK but also low income countries such as Botswana and South Sudan.

Relative to its population, per capita net migration to the UK in 2015 was 6.1 people per 1,000 inhabitants, which places it behind a number of other northern European countries.

So while the UK’s net migration rate does not stand out internationally, the debate about this number is markedly more heated than in other developed countries. Most European countries, even those with previously restrictive immigration polices such as Germany and France, have gradually recognised that immigration is an essential tool to address an ageing working population. Both countries have subsequently implemented more liberal immigration policies, particularly in regards to skilled labour migration.

Due to strong public pressure, British immigration policies have been presented, for the most part, as a way to prevent demographic growth. Plans to reduce net migration by current and previous Conservative governments are presented as a strategy to stop the population from reaching supposed unsustainable levels that could lead to housing shortages or increase hospital waiting times.

Who is coming in and out

Drilling down into the overall net migration figures, they encompass four broad legal categories of migrants. First, British nationals, moving out and back into the country. Second, citizens of the European Economic Area (EEA), who have made up almost half of net migration to the UK since 2002. The UK government cannot legally place any limitations on these two groups.

This leaves the third group – non-EEA nationals – as the only ones that the government can currently restrict. The vast majority of non-EEA arrivals in 2015 were either students (204,000) and skilled professionals on work visas (166,000).

While students provide substantial financial contributions to British universities, skilled professionals fill vital gaps in the UK labour market. From a purely economic standpoint, it makes little sense to limit the migration of students and skilled workers – yet significant restrictions have been placed on both visa categories in recent years.

The fourth category is asylum seekers as well as people (spouses, children, parents) joining family members already settled in the UK. There were 44,000 asylum seekers and 38,000 people joining family members respectively in the year to March 2016. But these two groups are legally protected from immigration restrictions by international human rights norms that the UK adheres to.

What Brexit would change

Some politicians in the UK and some other economically advanced countries are under the mistaken belief that immigration policy can be used to control migration flows and subsequent net migration numbers. Those campaigning for Brexit heralded the UK’s departure from the EU as a chance to take back control, including control of how many European migrants could come to Britain.

But the push and pull factors that influence global migratory flows are often beyond the legislative control of national immigration policies.

The truth is that countries with strong economies attract migrant workers. Studies have shown again and again that employment is the single biggest driver of immigration to the UK, both for skilled and low-skilled workers. Should the UK withdraw from the EU single market in a “hard Brexit”, it’s possible that the changing economic circumstances and the devaluation of the British pound will serve to make Britain a less attractive destination for migrant workers from the EU and beyond.

Conversely, British skilled professionals and graduates could be motivated to seek career opportunities abroad. Recently both Ireland and Spain have illustrated that changing economic circumstances can radically shift net migration numbers from positive to negative in less than two years.

Ironically, Brexit may very well facilitate a reduction of net migration numbers just as its proponents claim. But this will not be because of reclaimed national sovereignty over immigration from the EU, but because the post-Brexit economic turmoil will simply make Britain less attractive for foreign and domestic workers alike.

Jan Semmelroggen receives funding from the Social Science and Humanities Research Council of Canada

This article was originally published on The Conversation. Read the original article.