This pre-registered report aims to reveal whether board gender diversity enhance bank performance in a bank-based (financial) system. Using publicly listed banks in the period (2010 2021), we plan to measure bank performance and adopt a difference in difference method for the analysis. We focus on whether Japan’s Corporate Governance (CG) code strengthens the advisory or monitoring roles of women on board (WOB) in the banking industry. We shed light on the effectiveness of Japan’s CG code, which aims to increase the number of female directors. Furthermore, we contribute to the discussion of “Who monitors the monitors?” in a bank-based system.
We examine whether bank performance is enhanced by the degree of gender diversity on boards, measured by the ratio of women on boards (WOB), the ratio of women serving as outside director on boards (outside WOB), and the Blau index (BLAU), in bank-based (financial) systems, following our pre-registered report (Sakawa et al., 2025). Using data on Japan's listed banks in the period (2010−2021), we analyze the relationship between women on boards (WOB) and bank performance. To focus on the effect of Japan's Corporate Governance (CG) Code, we also use a difference-in-differences method. First, WOB are not effective to enhance bank-performance for the examined period. Second, the role of outside WOB may be effective in enhancing bank performance. Third, the introduction of Japan's CG code has not strengthened the monitoring or advisory roles of both WOB and outside WOB. These findings answer the research question: “Who monitors the monitors in a bank-based system?” Overall, the results imply that outside WOB may be effective in functioning as monitors.