During the twentieth century, immigration policy has progressed along similar lines in all of the project countries. Though their modern political histories differ in many respects, they have all been affected by the two world wars, and by the same long-and short-run economic trends. This shared experience is fundamental for any comparison of our six immigration countries.

As countries with at least de facto immigration, they may affect each other through their policy decisions. If, for instance, one state closes its borders, potential immigrants may try to be admitted to a neighboring state instead, which then experiences more immigration and may react with stricter controls or perhaps even close its borders completely. Immigration countries may make use of techniques already applied in other countries, such as the manipulation of residence and work permits or the alteration or enforcement of legislation concerning deportation or citizenship. The six countries studied are thus not independent cases, for the simple reason that they establish and implement their immigration policy under the same economic and political conditions and under the influence of the same prevailing ideologies.

Liberalism and nationalism

From the middle of the nineteenth century until the First World War two ideologies of great importance for international migration existed uneasily side by side: economic liberalism and political nationalism. Great Britain was the standardbearer of liberalism and led the way in 1846 with a systematic free-trade policy.