What impact of the ECB's negative interest rates on banks and credit in the euro zone?

Research literature arrives at totally contradictory results for the effect of negative interest rates on banks' excess reserves on banks' behaviour and credit supply: Some believe (1) that negative interest rates on banks' excess reserves reduce banks' profitability, and therefore lead them to limit credit supply and even to increase interest rates on loans; Others (2) believe that negative interest rates on banks’ excess reserves encourage them to increase lending to reduce the level of excess reserves. We seek to determine with which of these two theories actual developments in the euro zone seem compatible. We conclude: That negative effects (reduction in credit supply) have not appeared; But that the positive effects (stimulation of lending) have not been very powerful. G. Eggertsson, R. Juelsrud, L. Summers, E.G. Wold (2019) “Negative Nominal Interest Rate and the Bank Lending Channel” NBER Working Paper No. 25416, January S. Demiralp, J. Eisenschmidt, Th. Vlassopoulos (2019) “Negative Interest Rates, Excess Liquidity and Retail Deposits: Banks’ Reaction to Unconventional Monetary Policy in the Euro Area” ECB Working Paper No. 2283, May