Governments often justify interventions into the financial system in the form of bail outs or liquidity assistance with the systemic importance of large banks for the real economy. In this paper, we analyze whether idiosyncratic shocks to loan growth at large banks have effects on real GDP growth. We employ a measure of idiosyncratic shocks which follows Gabaix (forthcoming). He shows that idiosyncratic shocks to large firms have an impact on US GDP growth. In an application to the banking sector, we find evidence that changes in lending by large banks have a significant short-run impact on GDP growth. Episodes of negative loan growth rates and the Eastern European countries in our sample drive these results.

Size matters in banking. In this paper, we explore whether shocks originating at large banks affect the probability of distress of smaller banks and thus the stability of the banking system. Our analysis proceeds in two steps. In a first step, we follow Gabaix [Gabaix, X., 2008a. The Granular Origins of Aggregate Fluctuations. Available at SSRN: http://ssrn.com/abstract=1111765] and construct a measure of idiosyncratic shocks at large banks, the so-called Banking Granular Residual. This measure documents the importance of size effects for the German banking system. In a second step, we incorporate this measure of idiosyncratic shocks at large banks into an integrated stress-testing model for the German banking system following De Graeve et al. (2008). We find that positive shocks at large banks reduce the probability of distress of small banks.