The main finding of this study is that the negative association between parental SES and NFSB during adolescence/young adulthood to a large extent was mediated by school performance, accounting for as much as 60% of the variance. These findings are consistent with the large body of literature that has demonstrated the parallel links of parental SES to school performance [11
] and NFSB, respectively [21
]. However, this is the first study to examine the mediating role of school performance. We found no evidence for the hypothesis that the vulnerability to NFSB as a function of family socioeconomic environment may be moderated by school performance. This indicates that, although children with low SES face greater risk of poor school performance and, due to this fact, are overrepresented among self-injuring youth, the effect of poor school performance on the risk of NFSB seems to be equal for all SES-groups.
Parental SES had no effect on the risk of NFSB over and above what was mediated by school performance suggesting that childhood disadvantages act on a later risk of NFSB primarily in terms of the pathway effect. This means that low SES children tend to have a higher risk of NFSB and that risk can be understood in terms of them performing worse at school than comparisons. Thus, parental SES contributes to the intergenerational transmission of health disparities by placing children on different pathways leading to different mental health outcomes. The poor school performance of low SES children may be seen as a prolongation of the disadvantage experienced by one generation in the lives of the next and different mechanisms induced from conception onwards, by both material and non-material parental disadvantage, may be involved in impeding an underprivileged child's ability to reach his or her academic potential. For example, recent studies suggest that low SES increases the risk of exposure to adverse circumstances surrounding gestation and birth (e.g. inadequate nutrition, toxic exposure and stress) [30
] that can negatively influence brain development [31
] and its cognitive function in several ways [33
]. Low parental SES is likely to be accompanied by financial strain and psychosocial adversity related to poor parental health, alcohol misuse, and family disruption [1
]. These circumstances can undermine the amount and quality of stimulation that the child receives at home which is necessary for his/her optimal cognitive development [34
]. Furthermore, it is more common for low-SES children to live in poor neighbourhoods and attend schools with high percentages of disadvantaged and low-achieving students [35
]. These environmental characteristics have been shown to account for some of the risk of underperformance on an individual level, over and above the effect of individual SES and cognitive abilities [36
Inter-generational disadvantage resulting in poor school performance and, in consequence in NFSB, can also arise as an effect of restricted access to human, social, and cultural resources that enhance educational outcomes. Parental education, which is correlated with SES [37
], may play a particularly important role in this regard. Although low educated parents may value education as much as high educated parents, they often lack the ability to encourage their children to value education. Low-educated parents are less able to provide their children with qualified help with homework, early training in behaviours and skills (e.g. literacy) that are valued by schools, and to encourage them to achieve the expected outcomes [38
]. As a result, low-SES children are less familiar with school culture, values and expectations and therefore less well equipped to achieve educational goals. Better-educated parents are also more informed regarding strategic educational choices and better equipped to communicate with teachers [3
]. These characteristics allow the parent to closely monitor the child's performance and be proactive in preventing academic failure.
The differences in school performance between low and high-SES students may, to some extent, reflect inherited cognitive ability. There are, however, findings which indicate lower heritability of cognition among children from lower, rather than higher, SES backgrounds [42
], thus assigning greater importance for their cognitive outcomes to environmental factors.
School performance may be an important path because it is highly determinative of other exposures related to mental health. Early school performance is an important source for identity formation and the development of social roles [43
], for how peers rate each other's worth in the school's social hierarchy [44
], and clearly contributes to educational attainment and occupational opportunities later in life. By enhancing cognitive abilities such as critical thinking, and problem solving, school performance may also directly influence mental health. These are useful not only within the confines of the classroom but they also apply to situations outside the realm of the school and result in real-world
benefits, including better health [45
The presence of full statistical mediation suggests that school performance provides the key to understanding the mechanism through which long-term effects of parental SES on the risk of self-injury was transmitted and contributes to growing evidence that school performance may play a key role in the prediction of NFSB. It also extends previous findings by suggesting that school performance is not only an independent predictor of NFSB but also serves as a mediator between parental SES and self-injury. These results are in line with previous findings showing that a large proportion of the effect of childhood SES on adult health operates through its effect on education [18
]. However, whereas previous studies focused on educational attainment in adulthood, the current work suggests that educational stratification of mental health may already be discerned in the early stages of education.
The direct importance of school performance indicates that a child's SES is not fully determinative of later mental health problems expressed as NFSB. Thus, our findings suggest that when adequately meeting the academic needs of disadvantaged children, the negative health pathway caused by socioeconomic disadvantage may be counterbalanced or redirected.
The Hospital Discharge Register only includes NFSB cases admitted to in-patient care. It does not detect cases not seeking medical help after an act of NFSB or those who attended primary or specialized medical care, but were not admitted to in-patient care. Compared with a recent population-based health survey conducted in 2009 by the National Public Health Institute, the figures for NFSB are lower in the present study. In that survey, 8% of women and 3% of men aged 19-29, reported that they had, at least once, tried to take their own life [46
]. It cannot be taken for granted that the role of socioeconomic inequality in school grades is similar in cases of NFSB not reported to the medical system or in less-severe cases not in need of in-patient care.
The register-based design of this study did not allow for the control of mental health conditions that do not lead to inpatient care. Thus, the reverse association, meaning that poor school performance may be a consequence of mental health problems, cannot be entirely excluded.