In most industrialised countries, immigrant populations enjoy greater longevity than natives. And yet, people of foreign origin are often part of the most disadvantaged socio-economic classes, which are usually more exposed to mortality risks.

This epidemiological paradox is the focus of Jonathan Zufferey's doctoral thesis, which he has applied to Switzerland, using data from the Swiss National Cohort, based on the 1990 and 2000 censuses and on all deaths in Switzerland between 1990 and 2008. Conducted as part of the IP14 of the National Centre of Competence in Research LIVES with professors Michel Oris and Gilbert Ritschard of the University of Geneva as supervisors, this research was doubly relevant, as it dealt jointly with migration and inequality, two essential problems of the social sciences in general, and of LIVES in particular.

Jonathan Zufferey began by taking a closer look at the concept of foreigner in Switzerland, which covers a range of very different realities, depending on whether we are talking about first-generation migrants or subsequent generation migrants, and according to country of origin and status. However, it shows that all categories combined (with the exception of asylum seekers and illegal immigrants, who are not included in the data), people of foreign origin generally die later than the Swiss. Among men, only foreigners from Eastern Europe die earlier than Swiss citizens. As far as women are concerned, immigrants from Africa, the Middle East and Eastern Europe die earlier on average than Swiss women. However, the vast majority of immigrants come from southern or western Europe, and the tendency to die later is very marked in all these nationalities.

The same applies for causes of death: Jonathan Zufferey has discovered that foreigners seem to have a better resistance to risks than the Swiss. There is no overwhelming reason which explains this paradox. Among these causes, suicide appears to be rare across the board, as all foreign-born populations are at a lower risk of suicide.

Importance of bias

The results are particularly robust, as they are based on census data covering the whole resident population. By contrast, the existence of explanatory factors linked to bias cannot be ruled out.

In the United States and in European countries, research has already come up with several hypotheses in this regard to explain the phenomenon. Selection biases may occur when migrants enter and leave the country: so only the most resistant would attempt migration and remain long-term; the weakest would be less likely to attempt to migrate and are more likely to leave the country in the event of difficulties. Another bias could be linked to the data itself, if foreigners leave the country without notifying the authorities of their departure, which to some extent would make them statistically "immortal".

However, Jonathan Zufferey notes that the mortality differential also remains among the second generation, making the selection bias insufficient to fully explain the phenomenon.

Context and culture

The researcher thus also examines other lines of inquiry, such as the spatio-social context, providing a detailed analysis of mortality according to living environment. He observes that in the working-class areas, the longevity of foreigners remains greater than that of the Swiss. When these areas have associational, voluntary or community activities, the impact on health seems to be positive for nationals but remains neutral for immigrants.

Jonathan Zufferey's research shows that analysis should focus on intersections of the social structure by identifying the interactions which express accumulations or compensations of risk factors. By using data mining methods, he observes that it is in the most vulnerable social positions that the mortality gap between migrants and natives is the most marked.

In his conclusions, Jonathan Zufferey favours "an accumulation of explanatory factors" and partially credits the selection bias idea, although he states this is difficult to calculate. He develops the idea of a certain "migration culture", expressed via positive character traits, with "more open-mindedness" and "more drive" among those who attempt the adventure of migration and in their descendants. These people would seem to have a certain advantage when faced with risks, compared to the Swiss-born population.

Jury's comments

The thesis jury commended Jonathan Zufferey's "impressive work", the "scientific rigour", the "richness of the empirical approach" and the "ability to express his ideas clearly".

In response to professors Patrick Deboosere, of Vrije Universiteit Brussel, and Philippe Wanner, of the University of Geneva, who sought social policy recommendations, the doctoral researcher stated that Switzerland, due to its absence of ethnic ghettoes, could be a model for other countries. However, he underlined that mortality was just one public health indicator among others, and not necessarily the most nuanced, mainly because his study had not been able to take into account particularly vulnerable populations such as asylum seekers or illegal immigrants.

For his thesis directors, Jonathan Zufferey is an ideal example of these social science students "without an initial background in econometrics or statistics who go on to produce magisterial results", stated Prof. Gilbert Ritschard. The researcher thus gave "a wonderful illustration of an interdisciplinary demography", praised Prof. Michel Oris, adding that "the purpose of science is to advance, not give end points".

The research will continue straight away, as Jonathan Zufferey has already been taken on as a post-doctorate researcher for four years by the new National centre of competence in research On the Move. His future research will focus on the internal mobility of migrants, but he will also have access to unseen data which will enable the "salmon bias" to be controlled, i.e. how many foreigners go back to their home country to die. "Switzerland will be the first country to be able to conduct such research", enthused the young new doctor.