The Committee on Publication Ethics (COPE) has published a Discussion Document titled ‘What constitutes authorship?’ drawing upon a variety of sources: http://publicationethics.org/files/Authorship_DiscussionDocument.pdf
The COPE Discussion Document recognises that authorship in the legal discipline is:
still very much a product of the writing process, and usually by a single individual. Any other form of contribution such as generation of ideas, commenting on a draft, or technical assistance is listed in the Acknowledgments. Traditions in the humanities also differ from some disciplines in the social and natural sciences in terms of the relationship between supervisors and students in authorship with respect to graduate work. Frequently, students are sole authors of graduate-related research and supervisors and committee members are acknowledged for the supervision and mentorship that they have provided to the student authors.
The Report on the International Workshop on Contributorship and Scholarly Attribution held on 16 May 2012 at Harvard University discusses a range of issues relating to authorship and attribution: http://projects.iq.harvard.edu/files/attribution_workshop/files/iwcsa_report_final_18sept12.pdf