The crises in Syria and Libya have created large movements of people. This is the latest chapter in a long story of population movements.  Since the mid-nineteenth century international migration has been a constant theme in political, economic and social discourse.

Between 1850 and the beginning of the Second World War the largest flow of international migrants was from Europe to the Americas, as people travelled from the 'Old' to the 'New World' in search of a better life and opportunity. The reasons behind migration were no different from those that drive migrants today. Some were fleeing persecution and conflict while most hoped to find better work in a new country.

This process was heavily influenced by technological innovation. Nineteenth-century improvements in farming resulted in a labour surplus in southern and eastern Europe. Meanwhile, the development of large steam-powered ocean liners made transatlantic travel cheaper and safer. Together, these advancements paved the way for the migration of more than 50 million Europeans to the Americas.

There were other significant movements of people during the nineteenth and twentieth centuries. Africans migrated to the Americas while, under the British Empire, large numbers of Indians migrated to Africa, the Caribbean and south-east Asia. Many people from southern China also migrated to south-east Asia. Today the diaspora of people of Indian origin outside India is estimated at over 30 million people; the Chinese diaspora is in excess of 50 million people.

Globalisation, the declining cost of air travel and cheap communication have led to a surge in migration over the last quarter of a century. Since 1990 levels of international migration have risen by 50%. These population movements have three features.

First, while many think that immigration is largely into developed nations, 42% of the world's international migrants live in the developing world. Geographers from the Wittgenstein Centre for Demography and Global Human Capital in Vienna estimate that the largest region-to-region flow of migrants between 2005 and 2010 was from South Asia to the Middle East, driven by the construction booms in the Arabian Peninsula. The third-largest country-to-country flow of migrants was from Bangladesh to India (the greatest movement was from Mexico to the US).

While the US is home to one fifth of the world's migrant population, the largest share in the world, the list of countries hosting the most immigrants has some surprising constituents. Russia has the second largest share of immigrants in its population, followed by Germany, Saudi Arabia, the United Arab Emirates and the UK.

Second, adjusted for population growth, the global rate of migration has stayed roughly the same since the nineties. In 2013, international migrants comprised about 3.2% of the global population, a marginal rise from 2.9% in 1990.

Thirdly, the poorest countries are not the principal source of immigrants into rich, developed economies. Instead, it is fast-growing, transition economies such as India, China and Mexico, with educated and mobile populations that are the biggest sources of immigrants into the West. The demand for skilled labour in the West has led to a 73% rise in the number of migrants with higher education entering OECD economies in the noughties, a large proportion from India, China and Philippines. Mexico and China top the list of countries with the largest number of emigrants to OECD nations, followed, perhaps surprisingly, by the UK and then India.

Trends in immigration change over time. For much of the twentieth century, for instance, the numbers immigrating to, and emigrating from, the UK were broadly in balance – net migration was broadly flat. Between 1964 and 1983 the number of people leaving the UK exceeded the numbers entering. Then, once again, from 1984 to the mid-nineties, emigration and immigration were roughly balanced.

Immigration into the UK accelerated sharply after 1997, partly as a result of two policy decisions. The new Labour Government eased immigration controls and decided not to impose transitional controls on migrants from the new EU member states of central and eastern European.

Together these measures contributed to a four-fold increase in net migration to the UK between 1996 and 2010.  In the last ten years net migration to the UK has averaged around 250,000 each year. These levels of net migration into the UK are unprecedented - in the ninety years to 1990 the UK saw a net exodus of people, as emigration exceeded immigration. Indeed, for much of the post-war period the so-called "brain drain" was a focus of great concern, as the media and policymakers worried that the UK was losing its "best and brightest" to other countries.

In the 20 years to 2010, India was the principal source of migrants to the UK, followed by the US, Pakistan and Bangladesh. Since 2004, the EU has been a growing source of migrants to the UK and non-EU migration has moderated. In 2014, about 250,000 EU residents moved to the UK, roughly five times the average flows seen in the ten years before EU enlargement in 2004. Migrants come to the UK largely for work and study, although there has been a recent fall in the number of overseas students entering the UK.

Among EU nations France was the biggest source of migrants to the UK between 1990 and 2010, followed closely by Poland, Germany and Ireland. Over the same period the most popular destination for Britons settling abroad has been Spain, followed by South Africa, Australia, the US and Ireland.

Germany has the largest migrant inflows in the EU, followed by the UK. However, foreign nationals make up a higher proportion of populations in smaller member states including Luxembourg, Cyprus, Latvia, Estonia and Austria.

Population movements are as old as human history. Differences in demographics, incomes, opportunity and stability continue to offer powerful reasons for people to seek a better life overseas. The big differences today is that cheap travel, porous national borders and rising incomes in emerging economies have significantly increased the proportion of the world's population that is able to travel.

Recent events in Germany illustrate the conflicting factors facing host countries. Last week German's vice-chancellor, Sigmar Gabriel, said his country could take 500,000 refugees a year, "for several years". Germany expects to receive 800,000 asylum seekers this year, four times as many as in 2014. But yesterday Germany imposed temporary immigration controls on its border with Austria in an attempt to slow the in-flow of migrants to an economy that the interior Minister described as being, "at the limit of its capabilities".

Talk of 'migrants' suggests a homogenous group, whereas they, are, in reality, as varied as the populations from which they come – from the American investment banker with earnings in the millions to the penniless refugee from Syria, from a research scientist from India to a construction worker from Bangladesh. For rich countries like Germany the decision over what sort of migrants to accept, and how many, is a complex and sometimes troubling question.

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