When contracts are incomplete, the property-rights theory of firms suggests that ownership of physical assets provides better outside options, which in turn strengthen the owner's incentives to invest in the enterprise. This approach is less suitable for human capital firms such as management consulting that lack physical assets. This article develops an alternative theory for integration that sheds light on the boundaries of human capital firms. In particular, when a relationship between parties includes large potential externalities, reducing the outside option of each party will be beneficial. Integration provides this reduction by blurring the contribution of individual parties within the firm, and thus lowering their independent market valuation. Unlike some results in the property-rights literature, the results here are robust to variations in ex post bargaining solution.