In the past few decades, Europe has been experiencing increasing immigration. One of the consequences of this trend has been a renewed attention for citizenship policy. Historically, the regulation on the matter of citizenship has been remarkably stable. On the contrary, today we witness an intense legislative activity which is quickly changing the rules of the game. But what is the direction of this change? And which are the factors that trigger it? While it is reasonable to assume that migration must be one of these factors, what is the actual impact of migration on change?   There are several ways to acquire citizenship: at birth, by naturalization, by marriage. Let us focus on citizenship at birth. The laws that attribute it are rooted in the bodies of common and civil law. Common law traditionally applies the jus soli principle, according to which citizenship is granted by birthplace: the child of an immigrant is a citizen, as long as he is born in the country of immigration. Civil law applies the jus sanguinis principle, which attributes citizenship by descent, so that a child inherits citizenship from his parents, independently of where he is born. Starting from these distant origins, during the 20th century – and especially after World War II – citizenship laws have gone through a slow process of  adaptation, in conjunction with the decolonization phase, the collapse of the socialist system, and the mounting pressure of international migration. This adaptation has accelerated, especially in Europe, in the past 10-15 years, to the point that today the majority of the EU-15 countries apply a mix of provisions.   Sociologists and political scientists have advanced several hypotheses concerning the determinants of citizenship laws dynamics, on the basis of case studies and non-quantitative cross country comparisons. Pressure from a large stock of migrants pushes toward a legislation that allows elements of jus soli, but also drives toward restrictions of the same principle in countries where it was originally applied. The combination of these forces should induce convergence toward a common mix of jus soli and sanguinis provisions for countries coming from different legal traditions.   In a paper with my colleague Chiara Strozzi, I compiled a data set on citizenship laws across the countries of the world for the postwar period and I coded this information in such a way to allow a panel econometric analysis on the determinants of these laws. As of 2001, 24% of the 162 countries included in the data set apply jus soli, 54% jus sanguinis, and 22% a mixed regime. Jus sanguinis is the most common regime, with 69% of the countries in Africa, 83% in Asia, and 41% in Europe. The United States, Canada and New Zealand are among those that adhere to the jus soli principle. The growing group where a mix of provisions is applied is particularly well-represented in Europe, with 56% of the European countries, including the formerly jus soli United Kingdom.   On the basis of this data set, we were able to establish how exactly citizenship laws are shaped by migration and by a number of additional economic and non-economic factors. Overall, we found that increasing migration provokes restriction in the regulation for countries with a jus soli or a mixed regime. On the other hand, for countries with a jus sanguinis origin which have experienced large immigration, there is evidence of a tendency toward adding jus soli provisions. This tendency to liberalize, however, is far weaker than the tendency to restrict exhibited by the first group of countries. Since liberal countries tend to restrict while restrictive countries tend to innovate but only very slowly, the overall evidence offers only mild support for the convergence hypothesis. If anything, over the entire sample, migration appears to be a factor that makes regulation more exclusive.   A rationale for these findings can be found in the trade off that natives face when making decisions about citizenship laws. If including migrants may involve costs, there are also costs associated with their exclusion. The balance between the costs and benefits of a change in regulation determines if the change is actually adopted. The evidence therefore suggests that natives, or the policymakers that represent them, perceive migration as a factor that worsens this balance.   Our study also reveals other determinants, perhaps even more important than migration, of citizenship laws. The instability of national border tends to prevent the adoption of jus soli elements, since it questions the ability to identify who actually belongs to the native population. The data confirm that this factor has played a crucial role, during the postwar decolonization phase, in shaping the legislation of former colonies toward jus sanguinis and that it is likely to exert a persistent, restrictive impact on the choices of the new countries which were formed after the fall of the Berlin wall.   Furthermore, countries with an older population and with more extended political rights tend to be associated with more diffused elements of jus soli. Surprisingly, a large welfare state does not appear to represent an obstacle to liberalization, despite the fact that the enfranchisement of migrants may aggravate its burden. Commonly employed variables capturing cultural traits, such as religious affiliation and ethnic diversity, do not appear to add explanatory power to our regressions. Finally Europe, and in particular Southern Europe, deviates from these general conclusions by showing a stronger tendency to liberalize which cannot be fully explained by the variables we selected.   Citizenship laws not only shape the design of broader immigration policies, but also interact with the workings of labor markets, affect welfare programs, influence demographic trends, and interfere with international relations. It is therefore important to understand how they are changing, and why.  Even if European countries have moved a few steps in the direction of convergence, within the enlarged European Union there is still substantial divergence and incoherence. The road toward the notion of a common European citizenship appears to be a long one.