Patents as a Spur to Subsequent Innovation? Evidence from Pharmaceuticals
研究了制药业中首个进入者的专利保护（市场独占期）如何通过非侵权替代品激励后续进入，发现独占期每延长一年，后续进入的药物增加0.2种，且对临床进步较小的模仿者影响更大。
This paper examines how an incumbent's patent protection acts as an implicit subsidy toward non-infringing substitutes. I analyze whether classes of pharmaceuticals whose first entrant has a longer period of market exclusivity (time between approval and generic entry) see more subsequent entry. Instrumenting for exclusivity using plausibly exogenous delays in the development process, I find that a one-year increase in the first entrant's market exclusivity increases subsequent entry by 0.2 drugs. The effect is stronger for subsequent entrants that are lesser clinical advances, suggesting it is driven primarily by imitation.