Optimal income taxation with a risky asset: the triple income tax

We show in a two-period world with endogenous savings and two assets, one of them exhibiting a stochastic return, that an interest-adjusted income tax is optimal. This tax leaves a riskless component of interest income tax free and taxes the excess return with a special tax rate. There is no trade-off between risk allocation and efficiency in intertemporal consumption. Both goals are reached. As the resulting tax system divides income into three parts, the tax can also be called a Triple Income Tax. This distinction and a special tax rate on the excess return are necessary in order to have an optimal risk-shifting effect.