Citizenship rests with territory at the heart of the definition of nation-state. If territory determines the geographical limits of state sovereignty, citizenship determines a state's population. Beyond these limits one finds foreign land, foreign sovereignty, and foreigners. Drawing the boundary within which some human beings are included and others excluded as foreigners, permitting some of them to acquire citizenship with certain conditions and others to lose citizenship, is a state prerogative that requires legal tools. In citizenship law, the two most important legal tools traditionally used to determine citizenship are:
- Birthplace, or jus soli, the fact of being born in a territory over which the state maintains, has maintained, or wishes to extend its sovereignty.
- Bloodline, or jus sanguinis, citizenship as a result of the nationality of one parent or of other, more distant ancestors.
All nations use jus soli and jus sanguinis in defining attribution of citizenship at birth. However, two other tools are used in citizenship law, attributing citizenship after birth through naturalization:
- Marital status, in that marriage to a citizen of another country can lead to the acquisition of the spouse's citizenship.
- Past, present, or future residence within the country's past, present, future, or intended borders (including colonial borders).
In eighteenth-century Europe, jus soli was the dominant criterion of nationality in the two most powerful kingdoms: France and the United Kingdom. The state simply inherited feudal tradition: human beings were linked to the lord who held the land where they were born. The French Revolution broke with this feudal tradition. Against Napoléon Bonaparte's wish, the new civil code of 1804 granted French nationality at birth only to a child born to a French father, either in France or abroad. This policy of jus sanguinis, representing a modern innovation, was not ethnically motivated; it simply reflected the fact that individual rights and family had become more important than subjecthood and state power. This French innovation was borrowed extensively and became the law in Austria (1811), Belgium (1831), Spain (1837), Prussia (1842), Russia (1864), Italy (1865), Netherlands (1888), Norway (1892), and Sweden (1894).
The British tradition of jus soli, on the contrary, was transplanted, unamended and unbroken, to Britain's colonies in North America (the United States and Canada), Europe (Ireland), Africa (South Africa), and Australia. It also influenced Portugal and Denmark until the Nordic countries adopted a common nationality regime in the 1920s.
Were a population and territory to match one another exactly, attributing citizenship on the basis of jus sanguinis, jus soli or residence would not make any difference. Citizenship law would concern the same population and would have the same juridical effects. Further, naturalization would be irrelevant. It is the case, of course, that the population and territory of a nation-state do not coincide. People migrate and, with respect to migration, one can distinguish broadly between two different types of countries:
- "Countries of emigration" are countries where part of the core population resides outside the national boundaries, a characteristic applying to the majority of European countries before World War II (with the exception of France), and Mexico since the 1930s.
- "Countries of immigration" are those in which the majority of citizens are immigrants or descendents of immigrants, or whose foreign populations have settled as permanent residents alongside a majority population that is perceived to have existed since time immemorial and is not descended from immigrants. The United States, Canada, and Australia and countries of South America are examples of the former, while the latter category includes France since the mid-nineteenth century and all other western European countries since World War II.
The legal traditions of jus soli and jus sanguinis were maintained with consistency and relative ease in the majority of these different countries until World War II. In countries of immigration such as the United States, jus soli allows the children of immigrants to acquire citizenship automatically. For continental European countries that were countries of emigration, jus sanguinis allowed citizens abroad to maintain links until their descendants lost touch.
Since World War II, however, citizenship laws have converged across all democratic states, due to the large increase in the scale of migrations across the world. In many continental European states, large-scale postwar immigration led to legislative changes so as to permit increasingly large segments of the population born in their territories, namely second-and third-generation immigrants, to access citizenship more easily. Elements of jus soli have been included in their jus sanguinis tradition that extends citizenship automatically at birth to third-generation immigrants (France since 1889, the Netherlands since 1953, Spain since 1990, and Belgium since 1992). For the second generation, in many countries, children born to immigrants on national territory are entitled to citizenship if the child (Belgium, Denmark, Finland, France, Italy, Netherlands, Spain, Sweden) or one of the child's parents (Germany) has lawfully resided there for a period of years.
Countries with nationality laws based upon automatic jus soli often attracted a number of immigrants into their territory, encouraging these countries to become more restrictive. For example, the United Kingdom's imperial and expansive conception of territory, combined with its jus soli tradition, involuntarily encouraged immigration. Just after World War II, all subjects of the British Empire had access to British citizenship simply by residing in the territory of the United Kingdom proper. Since that time, British legislation on nationality has undergone a swift and silent revolution away from the extended and automatic jus soli to a 1981 law that attributes citizenship only to children born in U.K. territory to parents with legal residence status. The legal residence of parents has also been included as a requirement in the Portuguese and South African laws.
The trend toward convergence in nationality laws concerns almost all advanced industrial countries, insofar as they share three basic characteristics: democratic values, stable borders, and a self-perception as countries of immigration rather than of emigration. The importance of these three conditions is confirmed when considering exceptions to this rule, such as Israel and Russia. In both countries, there is a dominant perception that many of their citizens reside outside their borders, and that the borders—indispensable for the definition of the soli—are not stable. Jus sanguinis thus remains at the center of both of their citizenship laws. But for all countries, regardless of their situation concerning migration or their level of development, there are two distinct lines of convergence in nationality laws. First, there has been a notable trend since the mid-twentieth century toward repealing provisions for the automatic acquisition of citizenship through marriage, a move motivated at the same time by the development of equal rights between men and women in nationality laws and by worries about fraudulent marriages with illegal aliens. Second, equal rights between men and women to transmit their citizenship to their children has produced the development of dual citizenship and more toleration of this phenomenon in many countries that formerly refused it (for example, Switzerland since 1990). What remains divergent are the rules for naturalization; that is, the processes by which foreign residents of the first generation access citizenship in a host country. States generally require a period of residence and knowledge of the language, and take into account a criminal record, but the details of these requirements still vary greatly, both in the formal requirements of nationality laws and in the practices through which these laws are enforced.