In the midst of a heated global debate about immigration policies, a natural population-level experiment from Switzerland may provide some timely and relevant data for policymakers. It suggests that immigrants who gain citizenship in their new countries go on to have improved integration into the fabric of that country.

Obviously, establishing cause and effect here is very tricky: what if immigrants are only likely to apply for citizenship if they’re already well-integrated into society? Or what if only those immigrants who are well-integrated have their applications accepted? That makes it challenging to tell whether it’s the citizenship causing the integration, rather than the better integration encouraging to citizenship.

It’s an important issue to resolve, because it helps to clarify the question of what role citizenship plays in immigration policy. Some people argue that it should be an incentive, a benefit to be gained in exchange for working hard at integration. Others suggest that it should be a catalyst, helping immigrants become involved in their new societies.

Figuring out the accuracy of either of these positions is a challenge. It’s difficult for researchers to gain access to all the information that could influence whether someone applies for citizenship—this could range from having better information and financial resources to just being more motivated. Researchers also often can't find out why people had their applications rejected, especially when the reasons might be subjective or otherwise obscure.

In Switzerland, however, a number of municipalities decided citizenship applications by means of a secret ballot for around 30 years, a practice that ended in 2003. Local citizen voters were given leaflets with details about the applicants, and those applicants who receive a majority “yes” vote became Swiss citizens.

The researchers focused on this setup because it helped to eliminate some of the most important confounding factors. By looking only at those immigrants who made the application, they eliminate the question of whether only certain kinds of immigrants have the motivation or resources to apply for citizenship: everyone they study is on a roughly equal footing.

The procedure itself also helps the researchers to be clear on what factors were involved in applications being refused or accepted, because they’re able to access the exact information the voters were given.

For close calls, where the application was barely rejected or barely accepted by voters, there’s good reason to think that the borderline decision was basically random. “Lucky applicants who are narrowly approved and unlucky applicants who are narrowly rejected are similar on all confounding characteristics,” the authors write. Thus, the practice created two pools of immigrants who were largely similar but differed in whether they obtained citizenship.

When they surveyed these immigrants a decade later, they found that those whose applications were only just approved had significantly higher political integration than those who had only just failed. These people had increased political knowledge, were more likely to feel that they had a political voice, and were more likely to participate in politics through actions like voting, contacting politicians, or donating to political parties. This was consistent even for immigrants from different countries.

Because the survey was conducted in 2011-2014, which was a decade or more since the last citizenship votes in Switzerland, the researchers suggest that the results are picking up on genuine, long-term changes. It’s possible that immigrants might have a spike in their political participation after a successful application, but a temporary change is unlikely to have continued for a decade or more, they argue.

One important question to consider is whether the process in Switzerland has some characteristics that are likely to be different from other countries. For instance, perhaps something about gaining citizenship as a result of a vote by other citizens is really the catalyst for political participation rather than the citizenship itself. It’s possible that an immigrant who receives a positive vote on such a personally important matter might place more trust in the system and engage with it more than an immigrant who receives a negative vote.

Another potential objection to this study is that the researchers are wrong to assume that borderline cases are really all that similar. For the experiment to work, it has to be assumed that immigrants whose applications just failed by a hair’s breadth, and those who just passed, are the same in all important respects. When the researchers looked at characteristics like education levels, country of origin, or how long they’d lived in Switzerland, they did look the same. However, it could be the case that there were important details the researchers missed that actually made all the difference in political integration.

Given that social and political integration of immigrants is often something that policies explicitly aim to encourage, this is important information. Although a natural experiment like this would be difficult to find in other countries, future research will need to confirm whether the same effect seems to be consistent in different countries with different immigration procedures.

PNAS, 2015. DOI: 10.1073/pnas.1418794112 (About DOIs).