When habits are introduced multiplicatively in a capital accumulation model, the consumers' objective function might fail to be concave. In this paper we provide conditions aimed at guaranteeing the existence of interior solutions to the consumers' problem. We also characterize the equilibrium path of two growth models with multiplicative habits: the internal habit formation model, where individual habits coincide with own past consumption, and the external habit formation (or catchingup with the Joneses) model, where habits arise from the average past consumption in the economy. We show that the introduction of external habits makes the equilibrium path inefficient during the transition towards the balanced growth path. We characterize in this context the optimal tax policy.

We analyze the welfare properties of the competitive equilibrium in a capital accumulation model where individual preferences are subject to both habit formation and consumption spillovers. Using an additive specification for preferences, according to which the argument in the utility function is a linear combination of present and past values of own consumption and consumption spillovers, we analyze the circumstances under which these spillovers are a source of inefficiency. It is shown that consumption externalities have to interact with habits in order to generate an inefficient dynamic equilibrium. Finally, we characterize optimal tax policies aimed at restoring efficient decentralized paths.

We consider an economy where individuals use their standard of living based on past consumption in order to evaluate the utility arising from current consumption. We analyse how this process of preference formation affects the bequest motive. We show that habits (based on one's own past consumption) reduce the willingness of individuals to leave bequests, while aspirations (based on the standard of living of parents) make the existence of positive bequests easier. The long-run effects of both habits and aspirations on capital stock and on the amount of bequests depend on whether or not the bequest motive is operative. Copyright 2007 The Author(s). Journal compilation Royal Economic Society 2007.

In this paper, we show that consumption externalities are a source of equilibrium indeterminacy in a growth model with endogenous labor supply. In particular, when the marginal rate of substitution (MRS) between an agent's own consumption and the others' consumption is constant along the equilibrium path, the equilibrium does not exhibit indeterminacy. In contrast, when that MRS is not constant, the equilibrium may exhibit indeterminacy even if the elasticity of the labor demand is smaller than the elasticity of the Frisch labor supply. r

We 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-yougo 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. JEL classification codes: E21, E13, E62.