Do banks diversify portfolio risk? a test of the risk-cost hypothesis

proposes the risk-cost hypothesis that banks decide the number of loans by considering the costs arising from diversifiable portfolio risk. Thus, the banks do not minimize operation costs, but total costs including risk costs. This paper examines empirically whether the risk-cost hypothesis is valid, using financial panel data from Japanese banks from 1981 to 1994. Estimating the first-order condition of total cost minimization together with an operation cost function, we find that the hypothesis is supported. Dividing the sample into different types of banks, it is found that the hypothesis is valid for city and regional banks, but not for second regional banks.