Elizabeth Bates is Professor of Psychology and Professor of Cognitive Science at the University of California, San Diego, where she also directs the Center for Research in Language and the Project in Cognitive and Neural Development. She is a visiting scholar on a regular basis at the National Research Council Institute of Psychology in Rome. She has authored or co--authored more than 150 papers and nine books, including most recently, Rethinking Innateness (1996). Michael Tomasello is Co--Director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. His previous publications include Primate Cognition (1997), The New Psychology of Language: Cognitive and Functional Approaches to Language Structure (1998) and The Cultural Origins of Human Cognition (1999).