Banks' Stockholdings and the Correlation between Bonds and Stocks: A Portfolio Theoretic Approach

In this paper, we analyze the optimal asset composition ratio of stocks and bonds for a bank taking into consideration the correlation between the interest rate risk and equity risk in the financial capital market using a portfolio model. The analysis reveals that in determining the asset composition ratio in Japan, the correlation coefficient between the interest rate and stock prices as well as the stock price volatility plays a more important role than the interest rate volatility. We also show that in the present circumstances, the stockholding ratios of most financial institutions in Japan are higher than the levels calculated from the model. It is suggested that when the market is exposed to severe stress such as a surge in stock price volatility or reversal of the correlation between the interest rate and stock prices, the stockholding ratios would be even more excessive than the levels obtained from the model.