ESRB Analyzes Impact of Diversification in Sovereign Bond Portfolios

In the current Basel framework, a special treatment is reserved for exposures of European banks to domestic government bonds. Banks are not required to hold any capital buffer against their investments in sovereign bonds denominated in euro, which are considered de facto riskless. Furthermore, sovereign exposures are not subject to any concentration limits and can represent a large part of the capital of banks. Consequently, there are strong regulatory incentives for banks to hold disproportionate amounts of domestic sovereign debt for capital and liquidity reasons. Recently, the EU policymakers proposed the introduction of capital rules and diversification requirements for euro area government bond holdings, primarily to weaken the "doom loop" between sovereigns and banks. To inform the policy discussion, this paper analyzes the effects of possible responses of banks to the new diversification requirement. 

The analysis covers a sample of 106 European banks included in the EBA stress testing dataset during June 2013 to December 2015. These banks cover approximately 70% of banking assets in their respective countries and across EU. Sovereign exposures represent a large part of their total assets and are much larger than the capital of banks. First, the paper uses an independent component analysis to capture the dependence structure of sovereign risks of European countries and identify the common factors driving European sovereign CDS spreads. Next, the report analyzes risk and diversification in the sovereign bond portfolios of the largest European banks and discusses the role of “home bias”—that is, the tendency of banks to concentrate their sovereign bond holdings in their domicile country. Finally, the paper evaluates the effect of diversification requirements on the tail risk of sovereign bond portfolios and quantifies system-wide losses in the presence of fire-sales.

The analysis shows that reducing home bias by forcing banks to hold less concentrated sovereign portfolios may not necessarily lead to a decrease in portfolio risk for all countries or to a more stable banking system, especially during crises, when the dependence structure of sovereign risks should be taken into account. While the analysis of fire-sales indicates that diversification reduces fire-sale losses, this result must be tempered by possible network effects that may work in the opposite direction. If the regulatory authority introduces a large exposure regime, it may force connections and dependence between banks through joint cross-holdings, which represent an important channel of contagion in the presence of financial distress. Contagion can indeed occur because of fire-sales that result in devaluation of assets and contagion risk may thus increase with diversification. However, the paper concludes that estimating the most likely bank response to differing diversification requirements is an important task for future study.

Related Link: Working Paper (PDF)

Keywords: Europe, EU, Banking, Securities, Sovereign Exposures, Sovereign Risk, Concentration Risk, Sovereign Bonds, ESRB