Little is definitively known about how the interplay between `nature’ and `nurture’ brings about the (social) world as we know it. Nevertheless, huge amounts of knowledge on the importance of both nature as well as nurture amasses. Take for instance the incidence of schizophrenia, of which it becomes increasingly clear that a strong genetic component plays a role. Additionally, it is known that the incidence of a relapse into psychotic episodes is larger when relatives express great concerns to the schizophrenic, a mechanism sometimes referred to as ‘expressed emotions’.

One of the most consistent findings in social sciences is that of health differences between people in different social positions. As such, international research showed that the incidence of schizophrenia amongst immigrants is markedly higher than amongst the indigenous people.

Veling et al. set out to study differences in the (relative) risk of schizophrenic disorders between non-Western immigrants and native people in the Netherlands. To this extent, they took a strong approach, with some drawbacks. In collaboration with local general practitioners, psychiatrists and residents in psychiatry, they were able to have access to almost any onset of schizophrenia in the Dutch city The Hague. This was done during two periods: April 1, 1997–April 1, 1999 and October 1, 2000–October 1, 2002. By combining data on these cases with information on population characteristics, relative risks for people from different ethnic backgrounds could be compared.

It was found that the incidence of schizophrenic disorders was higher amongst immigrants from non-Western countries, such as Morocco, Surinam, Turkey, the Netherlands Antilles and other non-Western countries. Additionally, “the incidence rate was significantly higher for the second generation than for the first”. These findings are adjusted for age and sex.

On possible causal mechanisms, the authors point toward the social position of different immigrant groups. The incidence of schizophrenia was highest amongst Moroccan immigrants, who “have greater difficulties in their process of acculturation” than other immigrant groups. To the authors, this forms an indication for the “experience of social defeat and acculturative stress” as a possible causal explanation of the differences found.

This might very well be and certainly deserves further investigation. However, it appears to me all too easy to name only discrimination as an explanation of differences in schizophrenia amongst migrant minorities. It is well known as well that migrants in the Netherlands differ on other (social) characteristics as well, such as educational level, income and unemployment rate. This could easily have been taken into account, but has not been done. And what about lifestyle, eating habits, or other customs? What about the process of migration? The influence of genetic differences should not be excluded as well. None of these `compositional’ differences between different ethnic group have been taken into account, nor are they mentioned as hypotheses on causal mechanisms.

What I don’t understand as well is why the authors have calculated confidence intervals. The way I read the paper, the researchers have access to population-data for The Hague in the specified period. Why then inferential statistics are used, puzzles me: when information about all subjects in a population is available, we do not have a sample from which to infer to a (larger) population. This leads the researchers to reject that there is no difference between the incidence rates of the first and second period in which data was gathered, while in fact the incidence rate was slightly higher (2.2 per 10,000) in the second period than in the first period (2.1 per 10,000). This lead the authors to miss the possible hypothesis that the differences between the incidence rates of migrants and non-migrant in the Netherlands has increased. Combined with the heightened tensions between migrants and native people in the Netherlands in the recent years, this would form an excellent additional test for the discrimination hypothesis the authors propose, especially when data for more years will be gathered.

This paper is the first stemming from a larger research project with great societal relevance. Therefore, it is not very surprising that it is somewhat explorative in nature: differences are found but not explained. It can act as a strong starting point for continuing research, for many questions remain unanswered. It is to be hoped that not only the discrimination-hypotheses will be tested, but that the researchers will have a broader perspective on possible causal mechanisms.

VELING, W., SELTEN, J., VEEN, N., LAAN, W., BLOM, J., HOEK, H. (2006). Incidence of schizophrenia among ethnic minorities in the Netherlands: A four-year first-contact study. Schizophrenia Research, 86(1-3), 189-193. DOI: 10.1016/j.schres.2006.06.010