It has been a consistent finding that success in the school system depends heavily on the parents' SES [7-10]. Children from households with low SES do worse at school and achieve lower levels of education as adults than comparisons [11,12]. Educational outcomes, in turn, are critical to stratification processes affecting future employment opportunities and earnings potential [13]. Such stratification processes have been shown to have a strong influence on health outcomes. Better employment opportunities, safer work environments, and increased income, allowing investments in health and reducing economic instability, also enhance health and well-being. These work characteristics are, in turn, associated with a higher social position that is a predictor of good health per se [14]. Education is also associated with better health literacy and healthier lifestyles [15]. The benefits of education and the opportunities it provides are not confined to physical health but extend to mental health as well. A mounting body of literature indicates that education plays a major role in promoting mental health and preventing mental illness [16].

Despite this evidence, little emphasis has been placed on educational achievements during the school years as a possible link between low SES in childhood and the increased risk of poor mental health later in life. Although the hypothesis that the effect of childhood SES on subsequent mental health operates through its effect on education has been tested [4,17-19] and confirmed [18,19] in several studies, the research in this area has focused on the mediating role of adult educational attainment. The potentially cumulative effect of low parental SES and poor school performance also needs to be considered.

Suicidal behaviour among young people is one important mental health outcome where low parental SES, low intelligence (IQ) and poor school performance seem to increase risk [20-24]. In addition, findings indicate that rather than having a threshold effect, the risk of non-fatal suicidal behaviour (NFSB) in young people follows a socioeconomic and educational or IQ gradient [21,22,25,26]. Adolescence/young adulthood is a relatively healthy period, however the vulnerability to NFSB is most pronounced during this stage of life [27-29]. Moreover, NFSB rates among young people in Sweden have steadily increased over the past decade. In 2009, the rates of hospital admissions for NFSB in the 15-24 year-old group were 120 in every 100,000 men and 285 in every 100,000 women [29].

Descriptive data on NFSB and grade point average in relation to demographic data are presented in Table ​Table2.2. There was a gradient between parental SES, rates of hospitalisation and grade point average: the lower the parental SES the higher the rate of hospitalisation and the lower the grade point average. Men had lower grade point average and hospitalisation rate than women. Young people of mixed ethnicity had a lower grade point average and higher hospitalisation rate than young people with a Swedish and non-Swedish background. For the residency category, both grade point average and hospitalisation rate were quite similar between the groups.

Parental SES was associated with school performance [standardised beta coefficients ranged between -0.34 (children of unskilled workers had 0.34 units lower grade point average than children of higher-level non-manuals) and -0.13 (children of unskilled workers had 0.13 units lower grade point average than children of higher-level non-manuals); all at p < 0.0001], indicating that the condition required at the first step of mediation was fulfilled. The association between school performance and NFSB (HR = 0.39, 95% CI = 0.38-0.41) satisfied the requirements of the second step of mediation.

Table ​Table33 presents Cox regression models of the mediation analysis of parental SES, school performance and hospital admission due to NFSB. In model 1 (step 3 of mediation), adjusted for year of birth and gender, ethnicity and residency, the risk of NFSB increased with decreasing level of parental SES. Additional adjustment for grade point average in model 2 (step 4 of mediation) abolished this increased risk. These patterns were similar for both men and women. The results from the Sobel test confirmed that school performance significantly mediated the association between parental SES and the risk of NFSB (z = -38.9, p < .0001), accounting for 60% of the explained variance.

When interaction terms were add to the Cox proportional hazard regression model, the strength of the inverse association between parental SES and risk of NFSB did not vary according to school performance (grade point average). No significant interactions were found between the level of parental SES and school performance with respect to the effect of these variables on risk of NFSB (Table ​(Table44).

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 [31,32] 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 [1,3]. 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 [38-40]. 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 [3,8,41]. 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 [18,19]. 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.