Soc Psychiat Epidemiol (2010) 45:639–646 was related to less perceived racist group victimisation among the majority, while it related to more perceived racist victimisation for Turkish and Moroccan children (i.e. ethnic minority groups). Finally, the study by Monshouwer et al. [16] revealed an association between the ethnic composition of the school and alcohol use, but only in interaction with minority status. Ethnic minority students attending schools with a substantial proportion of minori- ties (10–30%) had less risk of episodic heavy drinking. Surprisingly, to our knowledge, no study so far addressed the question whether the ethnic composition of school classes inﬂuences students’ mental health. Hence, the current study aims to add to the knowledge on density effects and mental health by focussing on its importance in the school context of ethnic minority youth. Following the approach of Boydell et al. [5], and considering the large ethnic diversity in classes, we take into account minority status rather than ethnicity. In the present study we focus on the minority status, distinguishing between the domi- nant majority and (non-Western) minority groups. Method Sample and data collection In 2001/2002, the Netherlands participated in the Health Behaviour in School-aged Children Survey (HBSC), a large cross-national study on health in children and ado- lescents. In HBSC, a two-stage random sampling frame is used, which is described in the international protocol of the HBSC study [9]. First, out of a list of all schools for sec- ondary education in the Netherlands (a total of 587 schools, excluding schools for special education), a random sample of schools was selected proportionally within urbanisation strata. The response rate at the school level was 45% and this resulted in a total sample of 66 schools. Non-response could mainly be attributed to a lack of time (42%) or other research already going on in the school (37%). Schools in the most urbanised regions were somewhat less likely to participate. As argued by Vollebergh et al. [35] this is understandable due to the fact that universities in the Netherlands are situated in the most urbanised regions of the country and hence it is more likely that research is going on in these areas. No other differences between participating and non-participating schools were found. Within the schools for secondary education, from a list of all classes, one class per grade was randomly selected for participation. Only the ﬁrst four grades of secondary edu- cation were selected. Within each class, all pupils were asked to participate. The response rate of children within the schools was 95%. Absence of children during the data collection was primarily due to sick leave. This procedure 641 ﬁnally resulted in a total sample of 5,730 pupils from the ﬁrst 4 years of secondary education. Measures Internalising and externalising problem behaviour of pupils was measured using the Youth Self-Report [1]. The YSR is a questionnaire designed to be completed by adolescents, ages 11–18 years, and contains a total of 101 problem items. These items are scored as follows: 0 = not present, 1 = somewhat or sometimes true, 2 = very true or often true, on the basis of the preceding 6 months. The YSR can be scored on the total problem scale, the sum of all problem scales and eight syndrome scales. This research focusses only on the broad dimensions of internalising and exter- nalising problem behaviour, each consisting of the sum of all items in respectively three and two syndrome scales. The dimension of internalising problem behaviour consists of the scales, Withdrawn, Somatic Complaints and Anx- ious/Depressed. The externalising problem behaviour consists of the scales, Delinquent Behaviour and Aggres- sive Behaviour. The reliability and validity of the YSR are documented by Achenbach [1] and translated and validated for the Netherlands by Verhulst et al. [33]. In the current sample, the scales of internalising and externalising prob- lem behaviour displayed a satisfactory internal consistency, with Cronbach’s a of respectively 0.891 and 0.862 for the Dutch majority group and 0.890 and 0.867 for the ethnic minority students. Minority status was assessed by asking the pupils to indicate their own and both of their parents’ country of birth. Those reporting that either she/he or one of his/her parents was born in a foreign country belonging to the category of non-Western nations were scored as non- Western minorities, according to the Dutch deﬁnition [27]. The ethnic background of these minorities varied. In the total sample (including majority students), 2.9% were Turkish, 3.4% Moroccan, 3.6% Surinamese and 1.1% Antilleans, which together represent the four major non- Western immigrant groups in the Netherlands. Addition- ally, 5.4% came from a large number of other non-indus- trialised countries. Furthermore, as a measure of ethnic density, the pro- portion of pupils in class with ethnic minority status was calculated. Table 1 shows the distribution of students over classes with different proportions of minority students. Finally, individual level variables were added for gen- der, age, educational level, family afﬂuence and class size. For gender, a dummy variable was used with 0 = male and 1 = female. Age ranged from 11 to 18 (M = 13.9, SD = 2.10). In the Netherlands, pupils are on average 12 years of age when they start secondary education, and 16 years old by the end of the fourth grade. However, as pupils 123