Researchers in the fields of cognitive and language development have made less use of large-scale longitudinal designs and of person-centered approaches to data analysis than have researchers in the fields of social and personality development. It is argued that differences among domains of developmental psychology in the research methods employed reflect traditional differences in the goals of these fields. The study of cognitive and language development has sought primarily to discover the universals of development and their underlying mechanisms, whereas the fields of social and personality development have been more concerned with explaining differences in developmental trajectories and outcomes. A selective review of studies of cognitive and language development illustrates this connection between research questions and research methods. It is suggested that as research on cognitive and language development increasingly focuses on individual and group differences, scholars in these fields might profitably make greater use of large-scale longitudinal studies and of person-centered approaches to data analysis.