Valstad et al. (2026) presented important methodological advances in the study of indirect genetic effects in developmental psychology and psychiatry. By combining genetically informative family designs with measured genetic data, they addressed how sibling interaction and genetic nurture may contribute to developmental outcomes such as attention-deficit/hyperactivity disorder (ADHD) and educational performance. Their work highlights the importance of disentangling direct genetic effects from indirect genetic effects that arise through family relationships and shared environments. We discuss the conceptual implications of these processes for developmental psychology and psychopathology, and emphasize the interpretational challenges associated with correlated genetic and environmental influences. We note that heterogeneity in developmental processes may complicate statistical modeling approaches designed to estimate indirect genetic effects.