De l'utilité de l'impôt pour freiner l'effet de levier du « hors-bilan » des banques

The accounting standard IRFS have integrated the bulk of derivatives (the off-balance sheet in this paper) in the balance sheet of the banks at the cost of masking their leverage effect. We describe the bank activity and we show that these derivatives correspond to a financial intermediation function of the banking system, which supplements the standard lending role of banks. We show that the banking activity naturally increases the possibility set of the capital market that we define in general terms. As all innovations, this increase bears additional risk. The off-balance sheet appears in this framework as bringing a leverage effect without natural limit. We study what could be the role of different types of taxes: first, correcting and capturing the rents, second, limiting the systemic risk, third, mastering the ?growth of the feasible set?. A tax base consisting in financial liabilities (in absolute value) associated to positions on financial derivatives may be viewed as well-focused to limit the growth of the possibility set and therefore of the risk. Classification JEL : G21, G32, G38.