Discussion

In this paper we set out to address five key outstanding questions concerning the extent and nature of the overlap between conduct problems in childhood and early adult depression.

The first was to address current ambiguities in the relationship between conduct problems in youth as a precursor of adult depression, with some studies suggesting an association, ^{Reference Copeland, Shanahan, Costello and Angold3-Reference Kim-Cohen, Caspi, Moffitt, Harrington, Milne and Poulton5} but others failing to find a link. ^{Reference Althoff, Verhulst, Rettew, Hudziak and van der Ende6} It is probably as a result of this uncertainty that, despite increasing interest in early antisocial behaviours, depression is only rarely addressed as one of its outcomes. ^{Reference Pilling, Gould, Whittington, Taylor and Scott7} In our study using a large birth cohort, we confirm that conduct problems were strong markers of risk for subsequent depression: over 43% of young adults with depression in the ALSPAC cohort had a history of conduct problems in childhood or adolescence. Our study also demonstrates the potential public health importance of these findings by showing that PAFs for adult depression due to conduct problems were comparable to those due to early adolescent depression. This would imply that a considerable number of cases of young adults with depression could be prevented if early conduct problems were treated adequately and the association were causal. Effective treatments for conduct problems in youth ^{Reference Pilling, Gould, Whittington, Taylor and Scott7} might prevent some future depression, a hypothesis that our findings suggest should be tested empirically. Moreover, our study shows that the prediction from early conduct problems to young adult depression is largely independent of depression at ages 10 and 13 years. Of note, the prediction from conduct problems to young adult depression is cross-informant (from early parent- to later self-report), adding to the robustness of our findings.

Against this background, we turned to our second question: whether particular subtypes of conduct problems might carry an especially high depression risk. We focused on variations in age at onset, in light of extensive research on their roles as key markers of heterogeneity in antisocial behaviour. ^{Reference Moffitt8} More recently, distinctions have also been drawn within early-onset cases between individuals that persist in antisocial activities and those that do not. ^{Reference Odgers, Moffitt, Broadbent, Dickson, Hancox and Harrington9} To date, however, few studies have examined variations among these subtypes in risk for early adult depression. One possibility is that early-onset conduct problems (which typically show poor later outcomes) would also carry the strongest risk for depression; another is that conduct problems may be more strongly associated with depression if they are present in adolescence, when depressive symptoms themselves show a sharp increase. ^{Reference Thapar, Collishaw, Pine and Thapar27}

Of the few tests of these possibilities reported thus far, Odgers et al, ^{Reference Odgers, Moffitt, Broadbent, Dickson, Hancox and Harrington9} using data from the Dunedin longitudinal cohort, provide the closest match to our characterisation of child and adolescent conduct problems. They examined past-year rates of major depressive disorder in adulthood (age 32) for individuals following child and adolescent antisocial trajectories broadly similar to those examined here. In both men and women, risks of depression in the early 30s were markedly elevated for those on a ‘life course persistent’ antisocial trajectory, but were not significantly elevated for those on either childhood- or adolescence-limited pathways. Our findings were consistent with this pattern in finding much elevated rates of depression among young people with early-onset and persistent conduct problems, significantly higher than those in all other conduct problem trajectory groups. In addition, however, our findings suggested elevated risks for young people with both childhood-limited and adolescent-onset conduct problems. It is unclear at this stage how far these variations reflect differences in the ages at which depression was assessed (age 18 in ALSPAC, age 32 in Dunedin) or in other methodological factors. What is clear from both studies, however, is that conduct problems that begin early in childhood and persist into adolescence carry a particularly potent risk for subsequent depression.

Our third question concerned the development of depressive symptoms earlier in adolescence, and the extent to which young people with histories of conduct problems showed the typical profile of rising depression levels in the early to mid teens now widely established in population samples. ^{Reference Thapar, Collishaw, Pine and Thapar27} We found that children with conduct problems had significantly higher depression scores than their counterparts without conduct problems from late childhood onwards, but that the rate at which depression symptoms rose over the following years was similar in all groups. If replicated, these findings suggest that links between conduct problems and depression are established early and therefore through mechanisms that are independent of adolescent processes.

We tested for possible gender differences in depression risk, prompted by the ‘gender paradox’ hypothesis, ^{Reference Loeber and Keenan28} whereby girls with conduct problems are thought to show higher rates of comorbidity with depression than boys, despite lower overall rates of conduct problems. Consistent with past studies, we found that early adult depression was more common in females than males. However, we found no evidence that the rates of comorbidity between depression and conduct problems were higher in girls than boys. Importantly, these findings applied not only to early adult depression, but also to rising symptom levels in adolescence: the rise in symptoms of depression was stronger in girls (as indicated by the statistical estimates of the slope), but this effect was not moderated by conduct problem trajectory. Previous findings have provided mixed support for the gender paradox. ^{Reference Diamantopoulou, Verhulst and van der Ende29} We provide a relatively rigorous test of this hypothesis here, based on a cross-informant approach (parent-report for conduct problems and self-report for depression), and using two different instruments to ascertain depression at different developmental stages.

Our fourth question concerned the pattern of depressive symptoms across trajectories. Among young adults who met criteria for depression, the profile of depressive symptoms was generally similar irrespective of conduct problem history. We had predicted that irritability might differentiate young people with depression with conduct problem histories from those without, and found some support for this hypothesis, in that young people in the EOP group had significantly higher irritability scores compared with those in the low conduct problems class, controlling for depression severity. In a previous cross-sectional epidemiological study of adults, ^{Reference Fava, Hwang, Rush, Sampson, Walters and Kessler30} irritability was present in about half of respondents with lifetime DSM-IV major depressive disorder, and the presence of irritability was associated with earlier age at onset, higher comorbidity rates and increased disability. In addition, we have recently shown that among adolescents with depression, those with symptoms of episodic irritability were more likely to have comorbid disruptive disorder diagnoses. ^{Reference Stringaris, Maughan, Copeland, Costello and Angold11} In the current sample, levels of sleep and concentration problems also showed some differences between conduct problem trajectory classes. These were non-hypothesised findings, and thus require replication in future studies.

Our final question concerned the role of early family and child characteristics in the overlap between the two disorders that we observed. In the main, past studies of conduct problem/depression overlaps have focused on adolescent and later life-course risks that might be implicated in heterotypic continuities. ^{Reference Fava, Hwang, Rush, Sampson, Walters and Kessler30} We sought to extend these findings by focusing on factors evident earlier in development. We found that adjustment for early shared family adversities, child temperament, early cognitive skills and self-esteem resulted in only modest reductions in estimates of risk for young adult depression associated with childhood conduct problems. Our study cannot determine whether these associations reflect a direct causal impact of conduct problems on risk for depression or the operation of other shared risk factors, common to both disorders. It has been argued, for example, that the adverse consequences of conduct problems for children’s family and peer relationships, and for their skill development and academic performance result in a ‘failure trajectory’ that increases vulnerability to depression. ^{Reference Capaldi31} In addition, however, subsequent research suggests that the relationship between depression and conduct problems can largely be accounted for by shared genetic factors. ^{Reference Stringaris, Zavos, Leibenluft, Maughan and Eley32,Reference Tackett, Waldman, Van Hulle and Lahey33} Further research on these differing pathways would clearly be of importance.