AbstractWe study how the introduction of consumption externalities affects the efficiency of the dynamic equilibrium in an economy displaying dynastic altruism. When the bequest motive is inoperative consumption externalities affect the intertemporal margin between young and old consumption and thus modify the intertemporal path of consumption and capital. The optimal tax policy that solves this intertemporal inefficiency consists of a tax on capital income and a pay-as-you-go social security system. The later solves the overaccumulation of capital due to the inoperativeness of the bequest motive and the former solves the inefficient allocation of consumption due to consumption externalities. When the bequest motive is operative consumption externalities only cause an intratemporal inefficiency that affects the allocation of consumption between the generations living in the same period but do not affect the optimality of the capital stock level. This suboptimal allocation of consumption implies in turn that the path of bequest is also suboptimal. The optimal tax policy in this case consists of an estate tax and a capital income tax. The estate tax corrects the intratemporal inefficiency but generates an intertemporal inefficiency which is corrected by means of an appropriate capital income tax.