It is a great pleasure to write my first editorial, attempting to draw out the clinical relevance of some of the papers in this issue. I think a number of themes emerge in this issue, connected to the enduring problem of inferring underlying processes and problems from behaviour. We all know that behaviour is complex and messy, that the same behaviour may have many different underlying causes, and that the same core difficulty may have different behavioural manifestations in different individuals, at different ages or ability levels. These challenges, and some possible advances, are central to a number of papers in this issue.