Three researchers (two at Clemson and one the University of Arizona) recently announced the results of their study of the impact of California's board gender quota law (SB 826) on publicly traded [sic] firms headquartered in California.  They found that these companies experienced "a large negative stock market reaction" when the law was signed last year by then Governor Jerry Brown.  See Acknowledging Potentially "Fatal" Flaws, Governor Signs Board Gender Quota Bill.

The researchers also examined the costs of the law, finding bad news for smaller firms:

"The costs of board expansion are negligible for the largest firms but substantial for the smallest".  

They also observe that small firms may have a more difficult time attracting female directors. The Wall Street Journal was quick to note the study, labeling it in an editorial published on Saturday a "board diversity tax".

The authors of the study are Daniel Greene and Vincent Intintoli, assistant professors of finance in the College of Business of Clemson University and Kathleen Kahle, professor of finance at the University of Arizona. The study is available here.