When the U.S. Equal Employment Opportunity Commission filed suit against Sidley Austin in 2005 on behalf of 32 former partners, the demoted lawyers made an oddly humble argument. They said they were protected under the federal age-discrimination statute as mere employees because, despite their title, they lacked the “indicia” of partnership.

It wasn’t a bad point. With so many firms being run by centralized management teams, are most partners really employers or business owners?