After the French Revolution and due to a lawyer named François Tronchet, who resisted Napoleon and wanted to keep the old system, the French civil code decided that to be French, you needed to be born to a French father – where you were born on French territory, or abroad. It was an incredible break with the tradition of what we call jus soli – the law of the land, being born on the territory of the land. This revolution was adopted by force during the Napoleon conquest – when some territories of Europe adopted the same rule – or by free adhesion by many European States like Prussia and Russia, which adopted the same system that, in international law today, is sometimes called jus sanguinis: nationality attributed to birth, to fatherhood, like the family name. It was a will to create a means of national citizenship independent from the control of the State because you could travel and still transmit your nationality or your citizenship to your children.

The consequence of the impact of the French Revolution was that the world was divided between the countries that kept the old European tradition of jus soli, which was mainly the dominion of the British Empire: South Africa, America, Australia, Ireland, etc. In all these countries, citizenship or nationality was attributed by birth on the territory. Then you had the French impact that started in France and moved to Germany, Russia and Asia. So, you had two trends that co-existed in the middle or at the end of the 19th century.

When the phenomenon of migration developed, it had a major impact on nationality law. If everybody was living on the territory of their nationality and no foreigner would live in a foreign land, it would not matter if you had your citizenship by birth on the territory or by being born to a parent with citizenship.

But imagine you were born to an immigrant who had just moved to a new country, and in that country, citizenship is attributed to jus sanguinis, so you remain a citizen of the country of your parents. If you are born in a country where citizenship is attributed to birth on the territory, you immediately become a citizen of your country of birth. The country of emigration loses citizens because they move abroad. If this country attributes citizenship to birth on the territory, all the children of the citizens abroad are not citizens anymore, but if this country attributes citizenship to descent, the children of their citizens abroad keep the citizenship of the country of origin. Immigration has therefore had an impact on the citizenship of the children of immigrants.

And so, paradoxically, the first country in the world that had to confront the impact of its citizenship law because of immigration was France. France was the first country of immigration in Europe, but its new law, based on descent, made the children born to immigrants in France foreigners. And because they were foreigners, they did not have to perform military service, which was four to ten years for the young French male who had to obey French law and join the Army. Because of this inequality of duties, the French reinstalled a provision in addition to jus sanguinis that imposed citizenship at birth for people born in the country to parents already born in the country. For those not born in the country, citizenship would come at the age of majority. That was a mixture created in French law that became a trend all over the world.

The United States inherited the British tradition. So, if you were born in the US, you are American. But you also had Americans who went to live abroad. What happened to their children? The US Congress passed bills saying that if you are born to Americans abroad, you still have the nationality of your parents. That transmission stopped after two generations under US and British law. Yet, you also have a mixture, so the two traditions that created very different means of access to citizenship at the end of the 19th century have converged.

Today, I would say all nationality laws, even if they were created from different paths, have converged, mixing the attribution of citizenship to birth on the territory and birth to a parent – and not only to a father, because through the 20th century, women have acquired equality in the matter of nationality and they, too, transmit their citizenship to their children.

French women did not gain the right to vote until the end of the Second World War. There was also a feminist movement to gain this right to vote, to gain this independence of women’s citizenship, but they only won when the Conservatives noticed that so many immigrants were foreign males and they were marrying French women. These French women were becoming foreigners and their kids would become foreign kids. They were born to French mothers. That was very bad for the French population, and they were losing citizens, women and babies, who could have become French. So, the law of 1927 permitted French women to keep their citizenship, even if they had married foreigners, and to transmit citizenship to their children.

This movement developed all over the world. And today, in the majority of citizenship law, women have the same right to transmit their citizenship to their children. There is one big consequence: the development of dual and multiple citizenship. In the same couple, if the parents have different nationalities, they transmit at least two nationalities to their kids. If they themselves have inherited multiple citizenship, then they also transmit this multiple citizenship to their kids. So, more than the process of naturalisation – which is how an immigrant can become a citizen when he or she moves to a foreign country, applies for citizenship and the State makes a decision – marriage across the boundary of nation states, marriage with foreigners, has led to the important international phenomenon of dual and multiple citizenship.