Our results showed associations between 1) mother’s education and externalising but not internalising problems and 2) household income and both internalising and externalising problems but no evidence that higher CA of children modified any of these associations.
Our main aim was to examine whether CA modified the association of parental SEP with childhood emotions and behaviours. Specifically, our hypothesis was that children with higher CA would be more resilient to the adverse effects of low SEP on emotional and behaviour problems. There were few consistent patterns between the two cohorts when we examined associations by thirds of CA but nothing in these results to support our hypothesis. The relatively small sample size of each cohort may have limited our ability to test this hypothesis, but if our hypothesis were true even with relatively small sample sizes we would have expected to see greater effects of lower SEP in those with worse CA than in those with better CA and we saw no such patterns at all. Furthermore, it seems likely that any modification will be modest indeed if it cannot be detected in a sample of about 4,000 (ALSPAC).
It is possible that CA does not noticeably protect adolescents from the negative effects of growing up in a low SEP environment on their internalising and externalising problems. The adverse environments associated with lower SEP may be less amenable to modification by CA than other adverse factors that affect children’s mental health problems. Since lower SEP is associated with a wide-range of adverse environmental exposures, including reduced material resources, reduced parenting ability, poorer diet, poor housing conditions, poor neighbourhood amenities and poor education provision, it still remains possible that greater CA may improve resilience to some (but not all) of these but be insufficient to modify their combined effect on internalising and externalising problems. Furthermore, CA might not provide much benefit in situations where adolescents themselves do not have control over the stressful situations such as with chronic family socioeconomic hardship. If this were true CA might become a modifier of the association of SEP with mental health problems in later life, without a notable effect in adolescence.
Other factors should be considered for understanding why some adolescents from low SEP families develop internalising or externalising problems whereas others do not. It might be that personality and temperamental factors are more salient interactive factors in relation to affective and behavioural outcomes than a general cognitive indicator such as IQ.
Consistent patterns were observed in data from both cohorts with regard to associations between maternal education and externalising problems, and with regard to household income and internalising and externalising problems. We believe that it is important to investigate associations separately for these two SEP indicators [41
]. SEP is a comprehensive multidimensional construct involving financial, social, material and cultural circumstances [42
]. Household income and maternal education reflect different aspects of this construct, and they should therefore not be used interchangeably. Moreover they are likely to predict family processes and adolescent adjustment differently [44
]. Lumping specific SEP indicators together into a scale would obscure diverging associations of these with internalising and externalising problems and ignore the fact that they occupy different positions in aetiological developmental pathways.
Associations between household income and adolescent mental health have been reported more often previously [7
], than have associations between mother’s education and these outcomes. Our finding that mother’s education is associated with offspring externalising problems is in agreement with findings from previous studies [45
]. The lack of association with internalising problems was contrary to previous studies that had demonstrated associations of parental education (not necessarily the mother’s) with internalizing problems in adolescents [45
]. This discrepancy might be explained by our use of mother reports of internalising behaviour rather than self-reports by adolescents as in most previous studies, although most of the other associations reported here (including of maternal depression with outcomes) are consistent with existing literature.
There may be several reasons why adolescents from higher educated mothers have less externalising problems, including more favourable parenting styles (i.e. avoidance of harsh, humiliating and physical punishment), reduced exposure to stressful events within or in the surroundings of the family and less mental health problems in the parents. Potential education-specific explanations that have been suggested for inequalities in mental health reflect human capital more than economic capital, and include fostering of academic achievements of children, and promotion of social competence [44
Study strengths and limitations
The main strengths of this study are the use of two well characterised cohorts to examine our hypothesis. To our knowledge this hypothesis has not been tested in other cohorts. ALSPAC has a relatively large sample size and the TRIALS cohort provided a means of replicating any findings in order to provide more robust findings. We could only examine parent-reported measures of internalising and externalising problems. Problem scores based on parent- and child-reports in observational studies correlate only moderately [50
] and it is commonly recognized that multiple informants contribute unique information that can improve the overall measure of behavioural problems [50
]. For this particular study it means that the lack of moderation by CA refers to that aspect of adolescent problems observed by the parents (mostly the mother). In other words; adolescents’ CA does not seem to modify the association of SEP with behavioural problems of the child as reported by the mother. There is a possibility that results may have been different if the problems had been reported by the children themselves.
Associations presented in this study were adjusted for mother’s history of depression. Our justification for this is that mother’s depression could influence her educational attainment and household income as well as being associated with offspring behavioural or emotional problems, and could hence be a confounding factor. However, we acknowledge that mother’s depression could also mediate the association of parental SEP with offspring behaviours or emotions (i.e. lower SEP might cause maternal depression which in turn affects childhood behaviours or mood). In both cohorts adjustment for maternal depression did result in attenuation of the associations but we cannot ascertain from our data whether this is due to confounding or mediation.
Missing data in both studies was associated with most of the independent and dependent variables in the study. In ALSPAC, mothers of children for whom data could not be included in this study because of drop-out during earlier phases of the study or (item) non-response on core variables were lower educated on average (Pearson chi-square value 512.8 [df
= 4], P
-value < .001), had higher depression scores (mean depression scores 14.6 and 12.6 respectively, P
-test < .001), and had a relatively low family income (Pearson chi-square value 312.7 [df
= 4], P
-value < .001). Adolescents for whom information on IQ was available but who could not be included in the study because of missing data on covariates had lower IQs on average (mean IQs respectively 101.2 and 106.3, P
-test < .001). Adolescents for whom information on internalising and externalising problems at age 13 were available, but who could not be included because of missing data on covariates had higher scores on internalising and externalising problems (mean internalising problems 1.53 and 1.36, respectively, P
-test < .001; mean externalising problems 1.30 and 1.21, respectively, P
-test < .001). In TRAILS, non-participants at baseline were more likely to be from lower SEP families. Based on teacher ratings, there were no indications that non-participants at baseline had higher internalising or externalising scores [29
]. Adolescents who could not be included in this analysis because of missing data on covariates had on average lower IQ scores (mean IQs respectively 92.5 and 98.6, P
-test < .001), and had higher externalising problems scores at baseline but not higher internalising problem scores (mean externalising problems 0.26 and 0.24 respectively, P
-test < .001), had lower educated mothers (Pearson chi-square value 64.5 [df
= 4], P
-value < .001) and were from lower income families (Pearson chi-square value 127.4 [df
= 8], P
-value < .001) .
Whilst we cannot test this possibility, missing data would importantly bias our results only if the associations that we have found were in the opposite direction in those excluded because of missing data. Likewise, the failure to prove our hypothesis regarding the moderation of familial SEP associations with adolescent behaviours would be incorrect if in those with missing data there was a marked moderating effect of CA on SEP. Nonetheless we can think of no reasons why this would be the case.