Young et al. BMC Public Health 2011, 11:874 http://www.biomedcentral.com/1471-2458/11/874 are unrelated to suicidality, but this dramatically changes in the multivariate models, producing an appar- ent paradox; being Catholic generally reduces, while attending a Catholic school increases the risk of suicide. We can explain this contradiction by an intriguing inter- action; Catholic pupils attending Catholic schools report normal or slightly lower, but non-Catholic pupil attend- ing denominational (Catholic) schools report substan- tially higher levels of suicidality, not only while at school but also after school leaving, and vice versa for Catholic pupils attending non-denominational schools. It appears that pupils whose religion and school denomination are at odds are at greater risk. Although social background is a major influence, our results suggest that the school environment matters for mental health; more specifically, pupils with low levels of school engagement and involvement are more likely to attempt or seriously think about taking their own life or deliberately harm themselves. Irrespective of any the- oretical perspective and accepting the limitation of our measures, our most striking finding is that pupils who go to a school with a religious perspective incongruent with their own are approximately twice as likely to or attempt or think about suicide and four times more likely to self-harm. Durkheim’s theory and alterative explanations Durkheim’s theory of suicide proposes that egoistic sui- cides are increased when individual connections to society are weak and anomic suicides are increased by ‘mismatch’ between personal and societal norms and expectations. In relation to suicide-risk and self-harm, the ‘protective’ effect of school connectedness and the increased risk to pupils of a different religious orienta- tion from their school or the religion of the majority of pupils are two findings highly compatible with egoistic and anomic suicide perspectives respectively. However, there are alterative explanations for increased suicidality among ‘mismatch’ pupils. Parents sending their children to a school whose ‘ethos’ significantly differs from their religion may do so for reasons unrelated to religious belief; for example parents focused on their child’s scho- lastic success rather than wellbeing may choose a school based on school performance over other considerations. The inclusion of social background and parental care in our analyses may have reduced the impact of such unmeasured factors. We also cannot exclude the influ- ence of pre-existing mental health problems or risk fac- tors, although we included covariates that should have reduced the impact of such confounds. Ethnicity may play a role, since certain minority groups show high rates of self-harm [51] and some (non-Catholic) minor- ity parents prefer to have their child(ren) educated within a Catholic school. We explored this in further Page 13 of 15 (unreported) analysis and found ethnicity did not explain the high rates of suicidal and self-harm beha- viours in the ‘mismatched’ group. The West of Scotland has a history of sectarianism, and therefore the targeting and victimisation of pupils of a different religion [23] is another possible explanation for increased mental health issues among ‘mismatched’ pupils, although increased risk of victimisation can be interpreted as further evi- dence of the negative consequence of not sharing insti- tutional norms. Regarding the ecological transactional model of sui- cide, we replicated the relatively weak protective effects of the exosystem (neighbourhood) and stronger effects of the microsystem (school connectedness and parental attachment) on suicidal behaviour reported by Kidd et al. [27]. Further, the interaction between religion and school denomination is in-line with the importance that the ecological transactional model places on interactions between ecological levels. Our findings are also compati- ble with Joiner’s Interpersonal Theory of Suicide since it is plausible that, in combination with other risk factors, pupils who feel uncared for by parents, or who perceive greater exclusion by peers because of religious or other differences are likely to experience elevated levels of thwarted belongingness and perceived burdensomeness. Importantly because Joiner’s theory is a cognitive beha- vioural one, individual self-perceptions of isolation should be more important as risk factors than objective ecological measures of actual levels of exclusion and our findings are compatible with this more cognitive perspective. Several standard methodological caveats apply to our findings. It is always possible to omit important vari- ables by accident or through limitations of the dataset. At the other extreme, collinearity may be an issue. This may be particularly relevant to the school connectedness and school ethos measures because of the ephemeral nature and conceptual overlap of these constructs. To partially address this, we used measures derived through exploratory factor analysis, which may help to minimize, although not completely eliminate major collinearity problems [14]. Our study design is a practical one con- taining elements of both cross-sectional and longitudinal designs and thus imperfect; it does not fully track all suicide-risk outcomes and covariates at every time point - an ideal study would do so, allowing analysis of com- plex trajectories and their interaction with covariates. Despite the low prevalence of our outcomes, our study has adequate power to detect even small main effects, although power to detect interactions is modest. Never- theless, the consistency of findings across all three sui- cidality outcomes and the significant interaction found between religion and denomination using our suicidality measure suggests this is a reasonably robust effect, at