A large part of psychology concerns the study of individual differences. Why do people differ in personality? What is the structure of individual differences in intelligence? What are the roles of nurture and nature? Researchers in these fields collect data of many subjects and apply statistical methods, most notably latent structure modeling, to uncover the structure and to infer the underlying sources of the individual differences. Cognitive science usually does not concern individual differences. In cognitive models we focus on the general mechanisms of cognitive processes and not the individual properties. We believe that these two traditions of modeling cannot remain separated. Models of mechanisms necessarily precede models of individual differences. We argue against the use of latent structure models of individual differences in psychological processes that do not explicate the underlying mechanisms.