From Childhood Conduct Problems to Poor Functioning at Age 18: Examining Explanations in a Longitudinal Cohort Study

Abstract

Objective

Childhood conduct problems are associated with poor functioning in early adulthood. We tested a series of hypotheses to understand the mechanisms underlying this association.

Method

We used data from the Environmental Risk (E-Risk) Longitudinal Twin Study, a 1994–1995 birth cohort of 2,232 twins born in England and Wales, followed to age 18 with 93% retention. Severe conduct problems in childhood were assessed at ages 5, 7, and 10 years using parent and teacher reports. Poor functioning at age 18 years, including cautions and convictions, daily cigarette smoking, heavy drinking, and psychosocial difficulties, was measured through interviews with participants and official crime record searches.

Results

18-year-olds with versus without a childhood history of severe conduct problems had greater rates of each poor functional outcome, and they were more likely to experience multiple poor outcomes. This association was partly accounted for by concurrent psychopathology in early adulthood, as well as by early familial risk factors, both genetic and environmental. Childhood conduct problems, however, continued to predict poor outcomes at age 18 years after accounting for these explanations.

Conclusion

Children with severe conduct problems display poor functioning at age 18 years because of concurrent problems in early adulthood and familial risk factors originating in childhood. However, conduct problems also exert a lasting effect on young people’s lives independent of these factors, pointing to early conduct problems as a target for early interventions aimed at preventing poor functional outcomes.

INTRODUCTION

From Lee Robins’ seminal publication of Deviant Children Grown Up over 50 years ago¹ to a recent meta-analysis of over 30 empirical studies,² a wealth of evidence links early childhood conduct problems to poor adult outcomes. The long-term sequelae of conduct problems extend beyond mental illness to encompass poor functioning across other areas of life, such as education and employment, criminal offending, health and wellbeing, and social relationships.^3–5 Despite the burden that poor functional outcomes place on young adults and public services, little is known about why children with conduct problems fare poorly years later. We tested a series of hypotheses about potential explanations for the link between conduct problems in childhood and worse functioning in early adulthood, at age 18 years.

First, poor functioning in early adulthood may be the result of continuity in psychopathology from child to adult life, rather than a developmental outcome of conduct problems during childhood. Children who display conduct problems are at risk of continuing to exhibit behavioral problems into early adulthood⁶ and to develop other types of psychopathology, such as depression and anxiety.⁷ Psychopathology in early adulthood is associated with poor functioning,^8,9 raising the possibility that conduct problems during childhood predict later functioning because of its association with young-adult mental health problems.¹⁰ This hypothesis has rarely been tested because most studies investigating the young-adult sequelae of childhood conduct problems examine mental illness as an outcome alongside poor functioning. To the extent that the association is due to young-adult psychopathology, treating young adults who experience mental health problems will reduce their poor functioning.

Second, childhood conduct problems and poor young-adult functioning may be associated because they share the same risk factors. Children who grow up in socioeconomically disadvantaged families have greater rates of conduct problems compared to their more privileged peers,¹¹ and growing up in poverty is a major risk factor for poor functional outcomes.^12,13 Similar findings have been reported for children exposed to violence at a young age^14,15 or with parents who themselves display psychopathology, such as antisocial behavior or depression.^16,17 Socioeconomic disadvantage, violence exposure, and parental psychopathology could therefore explain why a childhood history of conduct problems is associated with poor functioning.

Third, in addition to well-established risk factors for childhood problems and poor outcomes in later life, there may be additional familial environmental and genetic influences contributing to their association. Twin and adoption studies show that genetic influences and, to a lesser extent, shared environmental influences, contribute to childhood conduct problems and young-adult functioning,^18–20 raising the possibility that both originate in the same familial risk factors. By comparing young twins growing up in the same family, who share the same environment and, in the case of identical twins, the same genes, it is possible to capture familial influences and determine the extent to which children’s conduct problems predict poor outcomes independent of latent familial risks. To the extent that the association is due to well-established familial risk factors and additional familial environmental and genetic influences, interventions aimed at improving the future functioning of children with conduct problems should address factors in a child’s family environment, for example through work with parents. To the extent that conduct problems in childhood predict outcomes above and beyond these factors, early, individual-level treatment of children’s problem behavior may improve future poor functioning.

We tested these potential explanations in a longitudinal prospective cohort of twin children who have been followed up to age 18 years. We investigated the extent to which childhood conduct problems predicted poor functioning in emerging adulthood. Our focus was on severe conduct problems with an early onset because they have a particularly poor long-term prognosis.²¹ The outcomes we examined reflect individuals’ functioning in emerging adulthood, across areas in which positive outcomes are critical for successful life-course development such as attainment, health, and social inclusion. In addition to testing whether childhood conduct problems predicted each outcome separately, we also tested the effect on the accumulation of poor functional outcomes, i.e. a cumulative index of poor functioning, because recent evidence has documented that individuals who function poorly in one area often experience difficulties in other areas, too.²²

METHOD

Participants

Participants were members of the Environmental Risk (E-Risk) Longitudinal Twin Study, which tracks the development of a birth cohort of 2,232 British children. The sample was drawn from a larger birth register of twins born in England and Wales in 1994–1995.²³ Full details about the sample are reported elsewhere.²⁴ Briefly, the E-Risk sample was constructed in 1999–2000, when 1,116 families (93% of those eligible) with same-sex 5-year-old twins participated in home-visit assessments. This sample comprised 56% monozygotic (MZ) and 44% dizygotic (DZ) twin pairs; sex was evenly distributed within zygosity (49% male). Families were recruited to represent the UK population of families with newborns in the 1990s, on the basis of residential location throughout England and Wales and mother’s age. Teenage mothers with twins were over-selected to replace high-risk families who were selectively lost to the register through non-response. Older mothers having twins via assisted reproduction were under-selected to avoid an excess of well-educated older mothers. At follow-up, the study sample represents the full range of socioeconomic conditions in the UK, as reflected in the families’ distribution on a neighborhood-level socioeconomic index.^25,26

Follow-up home visits were conducted when the children were aged 7 (98% participation), 10 (96%), 12 (96%), and 18 (93%). At age 18, 2,066 participants were assessed, each twin by a different interviewer. The average age at the time of assessment was 18.4 years (SD=0.36); all interviews were conducted after the 18^th birthday. Of the age-18 participants, 70.8% were studying for a degree at university or a vocational qualification, and 56.6% were working. 11.6% of participants were neither studying nor working.

There were no differences between those who did and did not take part at age 18 in terms of socioeconomic status (SES) assessed when the cohort was initially defined (χ2=0.86, p=.65), age-5 IQ scores (t=0.98, p=.33), or age-5 behavioral or emotional problems (t=0.40, p=.69 and t=0.41, p=.68, respectively). The Joint South London and Maudsley and the Institute of Psychiatry Research Ethics Committee approved each phase of the study. Parents gave informed consent, and twins gave assent between 5–12 years and then informed consent at age 18.

Childhood History of Conduct Problems

When the twins were aged 5, 7, and 10 years old, fourteen of 15 DSM-IV symptoms of conduct disorder were assessed (forced sexual activity was age inappropriate and thus not included) through mothers’ and teachers’ reports of children’s behavioral problems, using the Achenbach family of instruments and DSM-IV items.^27–29 A child was considered to have a given symptom if it was scored as being “very true or often true” (score=2) in the past 12 months, by either mothers or teachers, to enhance diagnostic validity.^30,31 To focus our analyses on children with moderate to severe conduct problems and following DSM-IV recommendations (APA, 2000), participants were categorized into those who had not versus had displayed 5 or more symptoms at the age 5, 7, or 10 assessment (n=307, 14.5%). More detail about the percent of children meeting this criterion at each age and across ages is provided in Supplement 1, available online. Findings were similar when using different symptom thresholds to categorize participants as having a history of conduct problems (see Tables S3 and S4, available online).

Poor Outcomes in Early Adulthood

We collected information on ten outcomes that reflected young adults’ poor functioning in areas critical to life-course development. Outcomes and their assessment are described in Table 1 and in Supplement 1, available online. Information on the majority of outcomes was ascertained at the age-18 interview; cautions and convictions were assessed through UK Police National Computer (PNC) record searches. NEET-Status (Not in Education, Employment or Training),³² parenthood, daily cigarette smoking, and self-harm or suicide were naturally dichotomous; all other variables were dichotomized. For variables with no predetermined cut-off (drinking, social isolation, and low life satisfaction), we defined poor functioning a priori as being among the 20% highest-scoring participants in an outcome. Findings were similar when using different thresholds (see Table S1, available online).

Young-Adult Psychopathology

During the age-18 interview, we assessed participants’ mental health over the previous 12 months including depressive disorder, generalized anxiety disorder, posttraumatic stress disorder (PTSD), alcohol dependence, cannabis dependence, and conduct disorder according to DSM-IV³³ and attention-deficit/hyperactivity disorder (ADHD) according to DSM-5 criteria.³⁴ Assessments were conducted in face-to-face interviews using the Diagnostic Interview Schedule (DIS).³⁵ We used a summary measure indicating whether participants experienced any of these mental health problems at age 18.

Risk Factors for Childhood Conduct Problems and Young-Adult Poor Functioning

Families’ socioeconomic disadvantage was defined at age 5 using a standardized composite of parents’ income, education, and social class,³⁶ divided into tertiles and reverse-coded. Child exposure to violence was indexed by child physical maltreatment and domestic violence by age 5, as previously described.^6,37 Briefly, child physical maltreatment by an adult was assessed for each twin during family visits using the standardized clinical protocol from the Multi-Site Child Development Project.^38,39 Interviewers coded the child as having not been, or as having possibly or definitely been, physically harmed on the basis of the mothers’ narrative, with inter-coder agreement on 90% of ratings (kappa=0.56). Adult domestic violence was assessed by asking mothers about their own violence toward any partner and about partners’ violence toward them during the 5 years since the twins’ birth, responding “not true” or “true” to questions about 12 acts of physical violence. The measure was dichotomized to reflect whether children lived in homes where there was any versus no adult domestic violence. Parental psychopathology was indexed by parents’ antisocial behavior and mothers’ depression.^37,40 Fathers’ and mothers’ history of antisocial behavior was reported by mothers when children were 5 years old, using the Young Adult Behavior Checklist,⁴¹ modified to obtain lifetime data and supplemented with questions from the DIS.³⁵ We combined reports about mothers’ and fathers’ behavior. Mothers’ major depressive disorder since the twins’ birth was assessed when the children were 5 years old according to the DSM-IV,³³ using the DIS.³⁵

Statistical Analyses

We tested whether a childhood history of severe conduct problems predicted each poor functional outcome at age 18 separately, and also a cumulative index of poor functioning, using Poisson regression models. We chose Poisson over logistic regression models for the dichotomous outcomes to obtain risk ratios,⁴² which are a more easily-interpretable measure of risk, particularly when outcomes are common. To test whether the effect of childhood conduct problems on functioning was accounted for by young-adult psychopathology and specific family risk factors, we included these as additional predictors in our regression models. We compared results across sex and observed similar patterns of results. We adjusted for sex in all analyses. Standard errors in all models were adjusted for clustering of twins within families. To test whether childhood conduct problems predicted age-18 outcomes within twin pairs growing up in the same family, we employed a discordant twin design, using fixed effects models with robust standard errors.⁴³ The resulting estimates indicate whether a twin with a childhood history of conduct problems is more likely to experience poor functional outcomes compared to their unaffected co-twin, accounting for family-wide environmental and, in MZ twins, genetic influences that may increase the risk for both conduct problems and young-adult functional outcomes. Stata version 14.1 was used for all analyses.⁴⁴

RESULTS

Do childhood conduct problems predict poor functional outcomes in early adulthood?

Participants with a childhood history of severe childhood conduct problems were at risk of poor functioning at age 18 years (Figure 1). Risks were elevated across all outcomes, ranging from incidence-rate ratios of 1.36 (for overweight) to 3.62 (for cautions and convictions). Poor functional outcomes were also associated with each other, so that participants with one poor outcome were more likely to display poor functioning in other outcomes (Figure 2). To capture this accumulation, we derived an index of cumulative poor functioning by summing poor outcomes. A childhood history of conduct problems forecast cumulative poor functioning in early adulthood (Table 2, Panel A). Closer inspection revealed that the majority of 18-year-olds without a history of conduct problems experienced none or only one poor functional outcomes (61.6%); few experienced two or three (28.7%), and fewer experienced four or more poor outcomes (9.7%). In contrast, only a minority of those with a history of conduct problems experienced none or one poor outcome (25.4%); most of them experienced two or three (41.7) or four and more poor outcomes (33.0%). This effect was not simply an artefact of childhood conduct problems predicting any one outcome particularly well, as indicated by analyses leaving out one outcome at a time (see Table S2, available online). We used the cumulative index of poor functioning in all subsequent analyses.

Figure 1.

Differences in functional outcomes between participants with a childhood history of severe conduct problems and those without. Note: Panel a presents the percentages of 18-year-olds who experienced a poor functional outcome among those with versus without a childhood history of severe conduct problems. Panel b indicates the incidence-rate ratio (interpretable as relative risk⁴²) for experiencing each outcome for those with a childhood history of severe conduct problems. The error bars represent 95% confidence intervals.

Figure 2.

Heat map illustrating associations between different poor functional outcomes. Note: Associations are indicated by tetrachoric correlations.

Table 2.

A Childhood History of Severe Conduct Problems Predicts Poor Cumulative Functioning at Age 18 Years

Panel A. Does a childhood history of childhood conduct problems predict cumulative poor functioninga at age 18 years?
Predictor	Bivariate model	Multivariate modelb
	IRR (95%CI)	IRR (95%CI)
Childhood conduct problems	1.96 (1.75,2.18)	- -
Panel B. Is the effect accounted for by young-adult psychopathology?
Childhood conduct problems	1.96 (1.75,2.18)	1.61 (1.45,1.79)
Young-adult psychopathology	2.22 (2.03,2.43)	2.06 (1.89,2.25)
Panel C. Is the effect accounted for by well-established risk factors?
Childhood conduct problems	1.94 (1.74,2.16)	1.40 (1.26,1.56)
Young-adult psychopathology	2.22 (2.03,2.43)	1.94 (1.78,2.12)
Socioeconomic disadvantage	1.41 (1.32,1.50)	1.23 (1.16,1.31)
Exposure to violence	1.50 (1.35,1.66)	1.13 (1.02,1.24)
Parental antisocial behavior	1.27 (1.21,1.33)	1.05 (1.00,1.11)
Maternal depression	1.31 (1.17,1.46)	1.04 (.95,1.15)
Panel D. Is the effect accounted for by unmeasured familial risk factors?
Predictor	Fixed-effects bivariate models	Fixed-effects multivariate models
	IRR (95%CI)	IRR (95%CI)
Childhood conduct problems	1.34 (1.13,1.59)	1.30 (1.10,1.54)
Young-adult psychopathology	1.48 (1.33,1.66)	1.47 (1.32,1.64)

Note: The N within each panel is restricted to participants with non-missing data on all variables included in the multivariate model within a panel. IRR=Incidence-rate ratio.

^aThe cumulative poor functioning measure is the sum of all poor functional outcomes

^bResults are adjusted for all other predictors within a panel and for participant’s sex.

Do childhood conduct problems predict cumulative poor functioning because of concurrent psychopathology in early adulthood?

Participants with a childhood history of severe conduct problems were more likely to experience psychopathology at age 18 years (IRR=1.75 [95%CI 1.57,1.96] p<.01). Psychopathology was associated with cumulative poor functioning (Table 2, Panel B). Including young-adult psychopathology as a predictor of poor young-adult functioning in our model reduced the effect of childhood conduct problems by approximately a third; however, it continued to forecast poor functioning, over and above young-adult psychopathology.

Do childhood conduct problems predict cumulative poor functioning because both reflect the same risk factors?

Participants who grew up in socioeconomic disadvantage, had been exposed to violence, or had parents with psychopathology were at greater risk of displaying conduct problems as children (IRR=1.66 [95%CI 1.40,1.98], p<.01 for socioeconomic disadvantage; IRR=2.24 [95%CI 1.71,2.94] p<.01 for violence exposure; IRR=1.68 [95%CI 1.54,1.84], p<.01 for parental antisocial behavior and IRR=1.62 [95%CI 1.26,2.08], p<.01 for maternal depression) and to experience cumulative poor functioning at age 18 years (Table 2, Panel C). These factors explained part of the association between childhood conduct problems and cumulative poor functioning, each reducing it by approximately a fifth. Taken together, familial risk factors accounted for approximately a further quarter of the association, beyond young-adult psychopathology. However, childhood conduct problems continued to predict poor functioning in early adulthood (Table 2, Panel C).

Do childhood conduct problems predict cumulative poor functioning because both reflect genetic and shared environmental influences?

Twin correlations of a childhood history of severe conduct problems (tetrachoric r_mz=.85 and r_dz=.62) and cumulative poor functioning at age 18 years (polychoric r_mz=. and r_dz=.4) were greater in MZ than DZ twins, indicating genetic influences. MZ correlations were less than twice the DZ correlations, indicating shared environmental influences. However, even twins raised in the same families and with the same genetic background differed in whether they had a childhood history of conduct problems and in their cumulative poor functioning, making it possible to test whether conduct problems predicted young-adult outcomes independent of familial influences. The prediction of cumulative poor functioning by childhood conduct problems was reduced when twins within the same family were compared to each other (Table 2, Panel D). However, the effect remained significant, indicating that childhood conduct problems predicted poor functioning independent of family-wide risk factors. We restricted our analyses to MZ twins to completely account for genetic influences and found that childhood conduct problems continued to predict poor functioning (IRR=1.32 [95%CI 1.11,1.57], p<.01). Taken together, these findings indicate that children with a history of severe conduct problems remain at greater risk for future poor functioning even after taking into account family-wide environmental and genetic risk factors that could explain the association.

DISCUSSION

Previous studies have shown that children with conduct problems are at risk of experiencing poor functional outcomes in adulthood. We extended this research in two ways: first, we examined the effect of childhood conduct problems on a cumulative index of poor functioning in emerging adulthood, reflecting outcomes across areas as broad as educational attainment, crime, health, and social relationships. Our findings show that 18-year-olds with a childhood history of severe conduct problems were more likely to experience difficulties across multiple spheres of adult life, more than a decade after they had first displayed conduct problems. The overall effect was modest but pervasive across young-adult outcomes that are not only relevant for individuals’ success during the transition to adulthood, but part of the foundation for health, wealth, and wellbeing across the life-course. Second, we examined explanations for the link between conduct problems and later poor functioning. Our findings revealed that children with conduct problems grew up to develop poor functional outcomes partly because they were more likely to experience psychopathology as young adults, and partly because they had been exposed to familial risk factors from early in life, both genetic and environmental. However, conduct problems forecast poor outcomes even after accounting for these explanations, indicating an independent, long-lasting effect. The findings have implications for our understanding of why children with conduct problems are at greater risk of poor outcomes, as well as for interventions aimed at improving young adults’ functioning.

Each of our hypothesized explanatory factors partly accounted for the association between childhood conduct problems and poor young-adult outcomes. It is well-established that conduct problems during childhood predicts conduct problems and other mental health problems in adulthood,^45,7 and here we showed that this continuity of problems across time may be partly responsible for the link between a childhood history of conduct problems and age-18 functional impairment. Our findings also suggest that childhood conduct problems and later poor functioning to some extent originate in the same underlying risk factors, both specific, well-established risk factors such as socioeconomic deprivation, and additional, unmeasured influences that we were able to capture through studying twins. These unmeasured familial risk factors may reflect environmental factors such as aspects of parenting or excessive stress that increase risk for childhood conduct problems and for poor functioning in adulthood. Familial risk may also reflect genetically influenced early-emerging behaviors and characteristics.

Our findings indicate that the presence of severe conduct problems during childhood signals poorer long-term outcomes independent of psychopathology in early adulthood^10,46. The association was also not explained away by familial risk. These findings suggest that conduct problems at a young age, themselves, have a lasting, negative, pervasive impact on young-adult outcomes. Conduct problems may do so by interfering with children’s ability to accumulate the human capital needed to become successful young adults. For example, children with conduct problems may be less well integrated into formal institutions and informal social settings that promote skills and positive functioning as children grow up, such as school or friendship groups of well-adjusted peers. Children with conduct problems may become involuntarily excluded from these settings if their behaviors evoke rejection from others⁴⁷, or they may withdraw from favorable settings by truanting or associating with peers who also display conduct problems.⁴⁸ Once children with conduct problems lose contact with settings that promote positive functioning, it may be difficult to catch-up in their development of skills needed to succeed in life, leading to pervasive functional impairment even years later.

The findings of our study need to be interpreted in light of some limitations. First, the E-Risk study participants were on the cusp of adulthood. It was not possible to examine negative outcomes that may unfold later in life⁴⁹ or life events and experiences such as marriage that may limit the negative effects of conduct problems or improve young adults’ functioning as individuals grow older.⁵⁰ However, poor functional outcomes in the transition to adulthood are informative because they are a foundation for individuals’ wealth, health, and wellbeing across the life-course. Second, our finding that childhood problems predicted poor adult outcomes within twin pairs does not establish causality. It is possible that factors not shared between twins, such as school experiences, explain the association. However, by taking into account environmental and genetic influences shared between members of a family, the twin comparisons rule out a powerful source of confounding of the association⁵¹. Fourth, because functional outcomes in young adulthood were all assessed at the same age, we were unable to account for the possibility that a poor outcome in one area may have influenced other poor outcomes to occur. For example, excessive drinking may have led to visits to the emergency department. Fifth, we cannot be certain that our results generalize to singletons. However, twins have been shown to be similar to singletons in the levels and development of behavior problems they experience during childhood.⁵²

Our findings have implications for future research and for interventions. First, future studies could benefit from adopting the approach of summarizing outcomes into a measure of cumulative poor functioning that captures the global impairment of young adults with a history of conduct problems. A cumulative measure empirically reflects the reality that poor functional outcomes are not restricted to any one specific indicator, but aggregate across outcomes.²² It may also help to identify global, underlying pathways that connect childhood conduct problems to pervasive poor functioning. Second, more research is needed to understand the processes through which bouts of severe problem behavior during childhood leave an imprint on children’s lives. Individuals who display stable high levels of conduct problems across development are at greatest risk for poor long-term outcomes,⁵³ but our findings add to a growing evidence base suggesting that bouts of early severe behavior problems still predict poor functioning over and above young-adult problems.^10,46 Third, although there is already strong evidence to support early intervention to limit childhood conduct problems, knowledge about the extent to which different explanations account for the link between childhood conduct problems and functional outcomes is important because it implies different strategies and targets for interventions. Our findings provide further support for comprehensive interventions that address familial risk factors for problem behavior and target children’s conduct problems from early in life onwards, to disrupt pathways of cumulative continuity and improve individual long-term outcomes.^54,55 Because the effects of conduct problems on later outcomes are pervasive, treatment has the potential to reduce economic burden across multiple public sectors,⁵⁶ such as the courts, the healthcare system, and social services, in addition to improving individuals’ wealth, health, and wellbeing across the life course.

Table 1.

Descriptive Information for Age-18 Functional Outcomes

Outcomes	Definition	Prevalence (%)
		Total	Boys	Girls
NEET	Not in education, employment or training (NEET) as self-reported in the age-18 interview	11.6	11.0	12.1
Parenthood	Any past live birth or current pregnancy (girls) or having caused a pregnancy that resulted in a live birth (boys) self-reported in the age-18 interview	2.6	1.1	4.0
Cautions and convictions	Official record of any UK cautions or convictions, beginning at age 10 through age 19	10.2	15.4	5.5
Overweight	BMI equal to or greater than 25, assessed at the time of the age-18 interview	25.8	22.7	28.7
Daily cigarette smoking	Current, daily smoking self-reported in the age-18 interview	22.3	23.5	21.3
Heavy drinking	High (among top 20% of participants) self-reported consumption of alcoholic drinks in a typical week at the time of the age-18 interview	20.3	25.6	15.5
Visits to emergency dept	Any visits to an emergency department within past year, self-reported in the age-18 interview	20.8	22.8	19.0
Suicide or self-harm	Any suicide attempts or incidents of self-harm between ages 12 and 18 years, self-reported in the age-18 interview	14.3	10.5	17.6
Social isolation	High score (among top 20% of participants) on a self-report scale reverse-coded to assess social isolation at age 18	20.0	24.0	16.5
Low life satisfaction	High score (among top 20% of participants) on self-report scale reverse-coded to assess low life satisfaction at age 18	18.7	19.9	17.6

Note: Further information on all outcomes is provided in Supplement 1, available online. BMI = body mass index.

Clinical guidance.

• The aim of this study was to examine why children with conduct problems experience functional impairment in early adulthood, at age 18 years.

• The findings indicate that functional impairment is due to a combination of the enduring negative effects of childhood conduct problems, associated family risk factors, and psychopathology in early adulthood.

• Clinical interventions that target children’s conduct problems and associated family risk factors have the potential to improve functioning in early adulthood.

Lay Summary.

• Children with conduct problems are at risk of experiencing difficulties as they enter early adulthood, such as unemployment, social isolation, and poor health, but it is unclear why that is the case. The findings of this paper suggest that poor outcomes at age 18 years partly result from a combination of long-term negative effects of childhood conduct problems, familial risk factors, and mental illness in early adulthood. Childhood interventions addressing these factors may help to improve children’s prospects of wealth, health, and wellbeing across the life course.