Background: Childhood conduct problems are a costly public health problem, five times more common in socially disadvantaged groups. Untreated, they have a poor prognosis, with increasing gaps between socio-economic groups, and high rates of subsequent criminality. The Incredible Years (IY) is a high-quality parenting programme as recommended by NICE for reducing conduct problems, and is widely disseminated in Europe. Many trials show IY to be effective, but the potential effects on social inequality of parenting interventions are unknown. This matters since some behavioural interventions (e.g. smoking cessation programmes), while beneficial overall, can widen inequality gaps. Since single trials and aggregate-level meta-analysis are ill-equipped for examining differential intervention (moderator) effects, we pooled individual-level trial data. Method: Individual participant data (IPD) meta-analysis of a near-complete set of randomised trials of European IY parenting programmes (N=1696; 15 trials eligible; 7% [1/15], data unavailable; 7% [1/15] lacked primary outcome). Children were aged 2-10 years (M 5.1; 30% [492/1651] ethnic minority; 58% [931/1614] low-income). Primary outcome was child conduct problems, using Eyberg Child Behavior Inventory (ECBI-I). Moderators were analysed using multilevel modelling with multiple imputation. Findings: IY led to an overall reduction in child conduct problems (13.5 points on ECBI-I, 95% CI 10.9 to 16.1). There was no evidence for differential effects by family disadvantage (poverty, lone/teen parenthood, joblessness; low education), or ethnic minority status. Interpretation: This world-first IPD meta-analysis of parenting trials, the largest pooled data set to date, found no evidence for differential effects by social disadvantage, suggesting IY is unlikely to widen socioeconomic inequalities in conduct problems. Furthermore, the programme may be an important tool for reducing social disparities and improving poor long-term outcomes in disadvantaged families, since follow-up studies indicate that benefits persist. Clinicians and commissioners can be reassured the programme is similarly effective for families from different backgrounds.