Parent–Child Sex Composition Impacts Long-Term Outcomes in Family-Based Behavioural Treatment

Introduction: Prior research by Temple et al. has shown that opposite-sex parent–child dyads demonstrate superior weight outcomes compared to same-sex dyads, but the analyses for all dyads were underpowered. This study replicates and extends those findings in a larger dataset of families provided with Family-Based Behavioural Treatment (FBT), examining whether dyad sex composition predicts change in zBMI and clinically meaningful weight outcomes at two-year follow-up. Methods: Mega-analysis of individual participant data pooled from 9 randomised FBT trials conducted between 1981 and 2013, with weight outcomes assessed at 6, 12 and 24 months. Clinically meaningful change was defined as ≥ 0.25 reduction in child zBMI and ≥ 5% parent weight loss. Results: Children, but not parents, in opposite-sex dyads showed larger reductions in zBMI and greater odds of achieving clinically meaningful change across 2 years. When examining the four dyad combinations separately, daughters in mother-daughter dyads had the lowest odds of achieving clinically meaningful reductions, whereas daughters in father-daughter dyads had the highest. Conclusions: Parent–child sex composition meaningfully influences children's long-term response to FBT. Opposite-sex dyads, particularly father-daughter pairs, show more sustained benefits. These findings suggest that dyad composition should be considered when administering FBT and may help inform strategies to enhance treatment effectiveness for families.