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When consumers concentrate their purchases at a single firm, firms that offer more products than their rivals gain market share for all their products. These spillovers induce firms to offer a greater variety of products rather than lower prices, and a concentrated industry with few large firms can arise if spillovers are strong enough. This article presents a simple model that illustrates this mechanism explicitly. The empirical analysis documents strong demand spillovers in the retail segment of the U.S. mutual fund industry, in which fees are nontrivial, families offer many funds, and the market is quite concentrated. Instead, spillovers are weaker, fees are lower, families offer fewer funds, and the market structure is more fragmented in the institutional segment.