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Recent empirical evidence provided by Bernard et al. (2010) and Broda and Weinstein (2010) shows that a significant share of product creation and destruction in U.S. industries occurs within existing firms and accounts for a significant share of aggregate output. In the present paper, and consistent with this evidence, we relax the standard assumption of mono-product firms that is typically made in dynamic general equilibrium (DSGE) models. We develop a DSGE model with multi-product firms and endogenous markups to assess the implications of the intra-firm extensive margin on business cycle fluctuations. In this environment, the procyclicality of product creation emerges as a consequence of firms' strategic interactions. Due to the proliferation effect induced by firm-level adjustments in product scope, we show that our model embodies a quantitatively important magnification mechanism of aggregate shocks.

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Multi-product firms dominate production activity in the global economy. There is widespread evidence showing that large corporations improve their efficiency by increasing the scale of their operations; this objective can be realized either by consistently investing in R&D or by expanding the product range. In this paper, we explore the implications of this fact by embedding multiproduct firms in a General Equilibrium model of endogenous growth. We analyze an economy with oligopolistic firms that carry out in-house R&D programs in order to achieve cost-reducing innovations. Market structure is endogenous in the model and is jointly determined by the number of firms and the number of product varieties per firm. Both economies of scope and scale characterize the economic environment. We show that the market equilibrium involves too many firms (too much inter-firm diversity) and too few products per firm (too little intra-firm diversity); moreover, we find out that the total number of products and productivity growth are inefficiently low under laissez-faire. The nature of these distortions is discussed in detail.

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