Abstract : The paper reviews the literature on disciplinary credit assignment practices, and presents the results of a longitudinal study of credit assignment practices in the fields of economics, high energy physics, and information science. The practice of alphabetization of authorship is demonstrated to vary significantly between the fields. A slight increase is found to have taken place in economics during the last 30 years (1978-2007). A substantial decrease is found to have taken place in information science during the same period. High energy physics is found to be characterised by a high and stable share of alphabetized multi-authorships during the investigated period (1990-2007). It is important to be aware of such disciplinary differences when conducting bibliometric analyses.