Gorton and Schmid (2000)
Gorton, Gary, and Frank Schmid. "Class Struggle Inside the Firm: A Study of German Codetermination." Working Paper, Wharton School Center for Financial Institutions, University of Pennsylvania, August 2000.
Finds that returns on assets and price/book ratio were lower for German firms where employees had a relatively higher degree of influence in management of the firm during the 1989-1993 time period. This is a very large and carefully-prepared study.
Link (NBER October 2000 draft): http://www.nber.org/papers/w7945