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To examine whether Japanese commercial banks exhibited economies of scale and economies of density at the time when the mega‐merger wave in Japanese banking began in the late 1990s. Since this merger wave has not yielded efficiencies, this analysis aims to shed light on whether banks, at the start of the wave, had reason to believe that larger banks would be more efficient.

Using a modified version of the translog cost function, the analysis estimates economies of scale and economies of density for Japanese city banks, trust banks, and regional banks. Then, the relationship between size and economies of scale/density and that between profitability and scale/density are explored using regression analysis.

Results suggest that larger banks (as measured by value of assets/loans/ deposits/investments, and number of employees/branches) were more likely to be in the decreasing/constant returns to scale/density region than smaller banks, The finding was statistically significant for all three types of Japanese banks. On average, city banks exhibited diseconomies of scale/density; trust banks exhibited constant returns to scale and increasing returns to density, and regional banks exhibited increasing returns to scale and density. This suggests that unions between city banks and either regional banks or trust banks may have been more likely to yield cost‐efficiencies, and raises questions concerning the efficiency motivations of the mega‐bank mergers. The findings further indicate that banks with higher sales were more likely to have exploited scale/density efficiencies, and that banks with higher net incomes were more likely to be in the increasing returns region.

This paper suggests that the mega‐merger wave in Japan in the late 1990s may not have been motivated by a desire for greater efficiencies through utilization of under‐utilized branch networks. Unlike other studies, this analysis differentiates between economies of scale and economies of density.