Soc Psychiat Epidemiol (2010) 45:639–646 to ascribe ethnicity. Furthermore, minorities from other Western countries could also have a relatively marginal- ized position in society (for example because of their skin colour or language barrier), and could be included in subsequent studies. Finally, no effects were found for the ethnic Dutch students. Although previous research has indicated that ethnic density effects also exist for the majority group members in contexts where they constitute a minority, it is not surprising that no effects were found. Of all ethnic Dutch students, over 90% were in a class where they constituted a large majority (over 70% Dutch). Further- more, this pattern will also be similar at the neighbourhood level, with the majority of the ethnic Dutch students living in neighbourhoods where most people share the same ethnic background. Implications Based on previous ﬁndings of ethnic density effects at the neighbourhood level [5, 10, 15, 17, 19, 20, 23, 32, 37], the present study examined the role of the ethnic composition of school classes on the level of externalising and inter- nalising problem behaviour. The ﬁndings of this study do not only contribute to the general knowledge on the effect of contextual factors on adolescents’ mental health, but can also contribute to the wider debate on the ethnic compo- sition of schools in the Netherlands [22]. Although in this debate—on ‘black’ versus ‘white’ schools—attention is paid to outcomes such as school achievement and inte- gration, students’ mental health can be considered of equal importance. Whereas ‘black’ schools are often equated with poor school achievement and are considered by some as a hindrance to the integration of minority students in mainstream society, our research ﬁndings indicate a ben- eﬁcial effect of such a school or class context on the mental health of students with an ethnic minority background, while negative effects on majority students have not been found. Other studies, such as the ones of Verkuyten and Thijs [34] and Peetsma et al. [21] also point to more positive aspects of schools and school classes with high ethnic density. Students may feel more at home and experience less racism and group discrimination in a more culturally diverse environment. Thus, based on the ﬁndings of the present study, an argument could be made on behalf of cultural diversity in schools. Future research In conclusion, the present study set out to test the ethnic density hypothesis in the school context. Previous studies on ethnic density effects have been carried out at the neighbourhood level, but given that adolescents spend a 645 great proportion of their time at school, the ethnic com- position of the school class can be expected to have an effect on adolescents’ mental health. Indeed, with respect to externalising problem behaviour, this expectation was conﬁrmed. However, more in-depth research into the role of ethnic density effects is warranted. The mechanisms underlying these effects should be addressed. Previous research has indicated that exclusion from local networks, the availability of cultural speciﬁc facilities, and experi- ences with intimidation and discrimination might be rele- vant factors in explaining the relation between the ethnic composition of the neighbourhood and mental health out- comes [37]. Translating this to the school context, includ- ing measures of the perceived social climate in class, of interpersonal relations between students, and measures of (perceived) discrimination could possibly improve our understanding of the density effect. In addition, how schools differing in their ethnic composition cater speciﬁ- cally to minority (or majority) students could be investi- gated. Furthermore, future research could focus on both the majority–minority distinction, as has been done in the current research, and ethnicity when examining the role of ethnic composition of the class. Finally, it might be inter- esting to study the role of the ethnic composition of the neighbourhood as well, and compare it to our ﬁndings within the school context. Although adolescents spend a large amount of their time at school, that does not disregard the role of other social contexts, such as the neighbour- hood. 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