There is concern whether established parenting programs for children’s conduct problems meet the needs of families with severe and complex mental health problems. For example, many children with conduct problems show comorbid attention-deficit/hyperactivity disorder (ADHD) or emotional problems, or have parents who are depressed, but families with such complex mental health problems typically seen in real life are often underrepresented in evaluation trials. We tested whether children with more severe conduct problems, and those with more complex mental health problems, benefit less from the Incredible Years parenting program, using individual participant data meta-analysis of randomized trials in Europe.

In 1,696 families from 13 children aged (child age 2−11 years; 37% girls; 58% low income; 30% ethnic minority; 98% mothers), we used moderator analysis within a multilevel model to test whether initial conduct problem severity, comorbid ADHD or emotional problems, and maternal depression would diminish intervention effects for children’s conduct problems.

The Incredible Years program reduced children’s conduct problems overall (Cohen’s d = −0.35), but more so in children with more severe conduct problems. There was no evidence that children’s comorbid ADHD and emotional problems changed the intervention benefits. Children of mothers with more depressive symptoms benefited more.

Children with more severe conduct problems derive greater, rather than lesser, benefits from a high-quality group parenting program, and comorbid ADHD and emotional problems do not reduce effects; maternal depression, rather than being linked to less child change, was associated with greater reductions in children’s conduct problems.