The paper adds a category to the list of possible negative steering effects of bibliometric indicators with a category for changes in credit assignment. The results of a longitudinal study of credit assignment practices in the fields of economics and information science are presented. The practice of alphabetization of authorship is demonstrated to vary significantly between the two fields. A slight increase is demonstrated to have taken place in economics during the last 30 years (1978-2007). A substantial decrease is demonstrated to have taken place in information science during the same period. A possible explanation for the demonstrated difference could be that information scientists have been much more aware of the bibliometric consequences of being first author compared to their colleagues in comparable fields (e.g., economics). This and other possible reactive tendencies of bibliometric indicators are presented and discussed.