How can we learn about developmental processes from cross-sectional studies, or can we?

Objective: Cross-sectional studies are often used in psychiatric research a s a basis of longitudinal inferences about developmental or disease process es. While the limitations of such usage are often acknowledged, these are o ften understated. The authors describe how such inferences are often, and s ometimes seriously, misleading. Method: Why and how these inferences mislea d are here demonstrated on an intuitive revel, by using simulated data insp ired by real problems in psychiatric research. Results: Four factors with m ajor roles in the relationship between cross-sectional studies and longitud inal inferences are selection of time scale, type of developmental process studied, reliability of measurement, and clarity of terminology The authors suggest how to recognize inferential errors when they occur, describe how to protect against such errors in future research, and delineate the circum stances in which only longitudinal studies can answer crucial questions. Co nclusions: The simple conclusion is that one must always use the results of cross-sectional studies to draw inferences about longitudinal processes wi th trepidation.