National views on the appropriate definition of the immigrant population vary from country to country. Despite this, it is possible to provide an internationally comparable picture of the size of the immigrant population, based either on nationality or country-of-birth criteria.
Nationality and place of birth are the two criteria most commonly used to define the "immigrant" population. The foreign-born population covers all persons who have ever migrated from their country of birth to their current country of residence. The foreign population consists of persons who still have the nationality of their home country. It may include persons born in the host country.
The difference across countries between the size of the foreign-born population and that of the foreign population depends on the rules governing the acquisition of citizenship in each country. In some countries, children born in the country automatically acquire the citizenship of their country of birth (jus soli, the right of soil) while in other countries, they retain the nationality of their parents (jus sanguinis, the right of blood). In others yet, they retain the nationality of their parents at birth but receive that of the host country at their majority. Differences in the ease with which immigrants may acquire the citizenship of the host country explain part of the gap between the two series. For example, residency requirements vary from as little as three years in Canada to as much as ten years in some countries. The naturalisation rate is high in settlement countries such as Australia, Canada, New Zealand and in some European countries including Belgium, Sweden and the Netherlands. In general, the foreign-born criterion gives substantially higher percentages for the immigrant population than the definition based on nationality. This is because many foreign-born persons acquire the nationality of the host country and no longer appear as foreign nationals. The place of birth, however, does not change, except when there are changes in country borders.
The data shown for the year 2000 come from a special census data collection covering almost all OECD countries. The foreign-born population data shown here include persons born abroad as nationals of their current country of residence. The prevalence of such persons among the foreign-born can be significant in some countries, in particular France and Portugal (repatriations from former colonies).
For a number of countries, reliable data on the foreign-born population are available only at the time of a population census. To make up for this deficiency, the OECD has developed estimates for a certain number of countries, applying two methods, the choice of which depends on the auxiliary information available (see www.oecd.org/els/migration/foreignborn).
For the foreign-born population, the data shown in the table under the 2000 column refer to 1999 for France; 2001 for Greece, Italy, the Slovak Republic, Spain; 2002 for Poland; the data under the 2007 column refer to 2003 for Germany and 2005 for Mexico. For the foreign population, the data shown in the table under the 2000 column refer to 1999 for France; 2001 for Australia, Canada, Greece; 2002 for Poland; those under the 2007 column refer to 2005 for France and Ireland.
The foreign-born population has increased in the past decade in all countries for which data are available. It is especially high in Australia, Canada, Luxembourg, New Zealand and Switzerland. Other countries, such as Spain, the Slovak Republic and Ireland, still do not report a high share of foreign-born population but have seen a spectacular increase in recent years. By contrast, the foreign population tends to increase more slowly, because inflows of foreign nationals tend to be counterbalanced by persons acquiring the nationality of the host country.