Standard horizontal foreign direct investment (FDI) models predict substitutability between FDI and exports in light of the proximity‐concentration trade‐off, nonetheless, empirical literature finds, almost invariably, a complementarity effect. We show that given the multi‐product nature of multinational enterprises both effects coexist at the firm‐level, with a substitutability for some products and a complementarity for others, explaining why the empirical substitutability relation has been so scarce even at the level of the firm. We use detailed French firm‐level data over 2002 and 2009 to show that the question of whether FDI and exports are complements or substitutes depends on whether the product belongs to the core competency of the firm and the size of demand in the destination market. We find evidence of the substitutability predicted by standard horizontal FDI models, taking place only for the best performing products of the firm and in high‐demand markets. In turn, vertical linkages and proximity advantages related to FDI's foreign presence generate exports of intermediates and products that are further away of the firms' core competency. This complementarity jeopardises the substitutability when aggregating all products of the firm, resulting on an average null net effect of FDI on exports in high‐demand countries.

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