640 there is a negative correlation between the incidence of mental illness and the size of an ethnic group relative to the total population [4, 10, 23]. This hypothesis is based on the assumption that living among members of one’s own eth- nic group can work as a buffer against some of the negative inﬂuences related to being a member of a minority group. One may feel more accepted and secure, and experience a stronger support network in an environment that includes a sufﬁcient number of other in-group members. A more general form of this hypothesis has been put forward by some authors [11, 36], who stated this could be a ‘group density’ rather than an ‘ethnic density’ effect, since evi- dence has been found for similar effects of characteristics other than ethnicity, given that these group characteristics are experienced as important. For example, group density effects have been found for religious afﬁliation [6, 25] and occupational grouping [36]. The ethnic density effect has mainly been studied in terms of the ethnic composition of neighbourhoods. Overall, studies on the effect of ethnic density largely conﬁrm the hypothesis that the risk of developing mental health problems in ethnic minorities decreases when the proportion of ethnic minorities in the neighbourhood increases [5, 10, 15, 17, 19, 20, 23, 32] while for majority members the reverse effect is found, i.e. their risk increases as the proportion ethnic minorities increases. Results of the early study of Farris and Dunham [10], conducted in the United States showed that while whites in general had lower psychiatric admission rates than blacks, the opposite was true in a largely black neighbourhood. In this area, with an extremely high concentration of black people, the admission rates of whites were unusually high while at the same time the admission rates of blacks were below average. These results were conﬁrmed by several other studies in the United States [15, 17, 23]. Research on the incidence of schizophrenia [5], deliberate self-harm [20] and suicide rates [19] among ethnic minorities in London also showed an effect of ethnic density, as well as a recent study on the incidence of schizophrenia in the city of The Hague, the Netherlands [32]. In the study of Boydell et al. [5], ethnic density was conceptualised as minority density, rather than speciﬁc ethnic density, thus underlining the assumption that the notion of similarity within more encompassing groups (like non-Western migrants) may be more important than the exact matching of ethnic groups. Closer inspection of studies revealed that the level at which the density effect is measured is crucial. A national study in the United Kingdom [7] for example showed no association between the proportion of an ethnic minority living in a particular area and their rates of admission for mental illness. However, as suggested by Halpern [11], this could be due to the level at which the effect of ethnic density has been analysed. In all of the previous studies the 123 Soc Psychiat Epidemiol (2010) 45:639–646 area of analysis was relatively small compared to the study of Cochrane and Bal [7]. Minority members may be dis- tributed unevenly within an area and it is likely that it is especially ethnic density in the direct environment of the person that plays a role in the development and prevalence of mental health problems. With respect to density effects among adolescents, only few studies have addressed the role of neighbourhood ethnic composition as a relevant factor. For example, Wickrama et al. [38] note that relatively few studies have investigated how factors at different levels (i.e. community, family, race) inﬂuence mental health and that research has mainly focussed on economic deprivation of the neigh- bourhood, rather than other aspects of the social environ- ment. In their study among black adolescents (aged 12–19) in the US, they found that risks of mental distress declined as the ethnic diversity of the community increased, thereby supporting the ethnic density hypothesis. Furthermore, Sampson et al. [26] report a large reduction in risk of perpetrating violence among young adults (aged 18–25) living in neighbourhoods with high versus low concentra- tions of immigrants, deﬁned as foreign-born residents. Hence, previous studies on the effect of ethnic density have predominantly focussed on adults rather than ado- lescents, and have generally taken the neighbourhood as a unit of analysis. In research on the mental health of ado- lescents, it makes sense to examine the role of the school context, assuming that it is the direct environment of the person that matters. Early and middle adolescents generally spend a large proportion of their time in their classes at school. For example, schools in the Netherlands are obliged to provide at least 1,040 h of class hours on a yearly basis, which averages over 26 h a week when excluding holidays. Additionally, there is much variation in the ethnic composition of schools and school classes. Therefore, the current research set out to examine the possible moderating role of the ethnic composition of school classes in the prevalence of internalising and externalising problem behaviour of ethnic minority and majority students. The effect of ethnic composition of schools and school classes has been studied with respect to a variety of out- comes, such as well-being at school [21], racist victimi- sation [34] and alcohol use [2, 16]. In general, these studies ﬁnd moderating effects for these outcomes, revealing dif- ferent reactions in majority and minority students that are supportive of the above-mentioned density effects. In their research in Dutch secondary education, Peetsma et al. [21] found that both Dutch majority and ethnic minority stu- dents in ethnically mixed classes reported greater well- being at school than students in more homogeneous clas- ses. Verkuyten and Thijs [34] found that for Dutch children (i.e. the majority), a higher proportion of Dutch students