This paper exploits enlargement of the European Union as a natural experiment to provide evidence for cluttering of the trade mark register in Europe. Enlargement increased regulatory uncertainty for pharmaceutical firms because the number of medical regulators that had to approve invented names for pharmaceutical products increased sharply at the time. The effects of this regulatory shock on pharmaceutical firms’ trade mark application strategies are studied using Difference-in-Differences and bias adjusted matching estimators. It is shown that enlargement had a significant and quantitatively important effect on pharmaceutical firms’ incentives to clutter trade mark registers with trade marks they are unlikely to use.

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