The conundrums inherent in the study of child and adolescent development are likely never far from the minds of the readers of JCPP . The complexity of development is flagged clearly in this issue. Included papers remind us that the link between associations (here maternal depression and child internalising problems) can change over short time periods. Even when the magnitude of associations stay consistent over time (here between the home environment and depression), there may be different aetiological processes underlying these links. Going forward, we should not lose sight of the significance of small changes in study designs, whether concerning the age range of participants, stipulated reporting periods and data analysis decisions (such as combining data from different age groups). The value of longitudinal data in addressing these questions remains paramount.