According to study findings, random assignment to the Fast Track intervention had a statistically significant and clinically meaningful positive effect on preventing childhood and adolescent externalizing psychiatric disorders and antisocial behavior, but only among the highest risk subgroup of kindergarteners. Intervention effects were detected as early as grade 3 and were robust through grade 9. Among the highest risk group in grade 9 assignment to intervention was responsible for reducing the risk of Cd cases by 75%, of ADHD by 53%, and 43% of all externalizing psychiatric disorder cases. Thus, the findings were both statistically significant and clinically meaningful. In contrast, the intervention had limited impact on children who were initially at only moderate levels of risk. The exception to this pattern was a significant main effect of intervention on self-reported antisocial behavior scores at the end of grade 9. These findings challenge the assumption that high-risk young children are impervious to change and join the growing body of descriptive literature on person × environment interaction effects in antisocial behavior (Caspi et al., 2002
; Dodge and Sherrill, in press
). Long-term, developmentally appropriate services that target child social-cognitive and cognitive skills, peer relationships, and parenting improved antisocial outcomes for highest risk youth.
One of this study’s strengths is its evaluation of the supplemental impact of the Fast Track intervention beyond standard community care for at-risk children. The control group in this kind of study cannot ethically be a “treatment-withheld” comparison, so these children were allowed to receive whatever interventions were available to them in their communities. Documentation indicates that routine services in special education, mental health, and juvenile justice for high-risk youth are costly and available (Jones et al., 2002
The analyses reported here indicate that these findings do not vary significantly across sex groups, across four geographic sites at which the intervention occurred, or across three cohorts of children. Given the high sample retention and few differences between children who remained in the study and those who did not, the study’s findings (likely chance) can be generalized to a large segment of high-risk American children. The robustness of this approach to intervention across these diverse groups of children improves its utility for dissemination across diverse contexts, although future studies should still test its generalizability.
A somewhat surprising finding was the developmental variation in outcome effects. That is, more favorable effects were found on CD and ODD diagnoses at grades 3 and 9, as compared to grade 6. We do not have a precise explanation for this finding, but two factors may be considered. First, the findings for grade 6 are significant for ADHD symptoms and effects for CD and ADHD diagnosis were trends, thus showing the same patterning, albeit less strong at grade 6. Second, it may be that the effects of intervention were somewhat disrupted by the transition to middle school and the decrease in intervention services after entrance to middle school. These findings point out the need in prevention trials to follow participants past the middle school period and into later adolescence where similar developmental trends have been found in other prevention trials (Vitaro et al., 1999
Although the effects on the highest risk group are robust and favorable, given the high cost of this intervention (about $58,000 per youth over 10 years; Foster et al., 2006
), one may wonder whether the benefits will outweigh the intervention costs. Cohen (1998)
estimated that high-risk youth cost society $1.2 to 2 million each in rehabilitation, incarceration, and costs to victims. At an intervention cost of $58,000 per youth, an intervention must reduce such costly outcome cases by just three to five percentage points to be cost beneficial. The effect size among the highest risk group in the present study indicates that CD diagnoses were reduced by 16 absolute percentage points and overall externalizing psychiatric cases were reduced by 20 absolute percentage points, suggesting that large economic savings may ultimately accrue with this intervention if it is directed to the highest risk group.
These same analyses point toward a suggestion for how to maximize the cost-effectiveness ratio for preventive interventions such as Fast Track. The cost-effectiveness ratio increased linearly with increasing level of initial risk. Thus, it follows that limiting selection to the highest risk group would maximize the investment in prevention.
Several limits of the present study temper the implications of the findings. First, whether the effects of intervention will last into adulthood is unknown. Because the intervention was still being implemented at the time of the outcomes reported here, it is plausible that the temporary scaffolding, support, and monitoring provided by the intervention staff was responsible for the interruption of antisocial behavior and that, once the intervention ceased, the effects would dissipate. The Fast Track intervention terminated 1 year after the current outcomes were measured; thus, continued follow-up of these youth after intervention ends in adulthood is crucial. Second, as schools were selected for high rates of local crime and poverty, these findings require confirmation in schools with average or lower levels of crime and poverty.
Third, because the Fast Track intervention includes a variety of services in several domains at different ages (just like the Olds et al. [1998 nurse-practitioner home-visitation program), it is not known whether the entire program is necessary or which components of the intervention are most crucial to positive outcomes. The comprehensive intervention addressed child social-cognitive and cognitive development, peer relationships, and parenting across 10 years. It is plausible that one or more components accounted for most of the impact. Mediation analyses may help sort out the most potent components, but future factorial experimental studies are needed to parse the program into components. Finding the minimally essential components may also bring the advantage of enhancing the cost-effectiveness ratio of the intervention.
Fourth, the positive effects of intervention on ADHD criterion counts may seem confusing given that ADHD is likely biologically driven. The reported ADHD scores, however, are a function of the rater’s (i.e., the primary caregiver) perspective of the child’s behavior. Raters may implicitly refer to conduct problems when responding to questions pertinent to ADHD, and so favorable intervention for conduct problems may reduce the perceived severity of ADHD symptoms. The effect of conduct problems on raters’ perceptions of ADHD symptoms may also be an important issue for the psychiatric assessment of ADHD. In addition, unlike the DSM, our criterion for ADHD did not require early age of onset (because, by design, we assessed current symptoms only), so the possibility that current conduct problems could skew a perceiver’s rating of ADHD symptoms was enhanced.
Finally, although the Fast Track intervention had little effect on externalizing disorder in the moderate-risk group, it is inappropriate to conclude that no intervention with these children would be efficacious. During the years immediately following the initial screening, when comprehensive and intensive prevention services were provided to the entire high-risk sample, the program had significant main-effect impact on the entire group of participants (Conduct Problems Prevention Research Group, 2004
). Over time, the design of the prevention program was to gradually phase out group interventions and focus more intensive individual services (home visiting, academic tutoring, mentoring) on those who demonstrated the greatest need. The interaction effects with risk status seemed to emerge over time. Hence, it is possible that this focus on intensive, individualized intervention may have led to diminished effects in the moderate-risk group, or that the intensive individualized service model was most useful only to the highest risk youth and families. It is also possible that the moderate-risk children require different kinds of support than were given in this program. Yet another possibility is that the outcome measures reported here did not fully assess the adaptation of the moderate-risk group. Their rates of serious problem behavior as measured here may have been too low to detect an effect of intervention; 24% of the total sample scored within the normal range on the teacher ratings of externalizing in kindergarten. It is unclear whether different measures would be more sensitive to intervention effects for this group.
These findings support the importance of screening when selecting children for comprehensive preventive interventions, Assignment to intervention had a clearly favorable impact on children in the highest 3% of the population in terms of risk but little positive impact on the moderate-risk children, although the relation between risk level and intervention effectiveness was seen to be linear and gradual. Although there is no sharp cutoff for effectiveness, practitioners faced with selection questions for this kind of comprehensive and costly intervention would likely maximize cost-effectiveness by focusing on the highest risk group. Clinical researchers in search of higher levels of statistical power would be wise to resist increasing sample sizes merely by reaching farther down in risk levels to obtain larger samples When intervention studies allow large enough sample sizes, researchers should test for intervention effects that interact with initial risk level.
In conclusion, this study demonstrates for the first time that early intervention can reduce long-term, serious conduct disorders in high-risk children. The fact that the intervention extended over a greater length of time than previous interventions may account for its success, but this is an issue for future inquiry. This study challenges the hypothesis that high-risk children are impervious to intervention, and it provides hope for these children and the practitioners who are charged with intervening with them.