Effects of Patents on R&D,Economic Growth, and Welfare

本研究构建了一个结合质量阶梯模型和品种扩展模型的内生生长模型。家庭的效用来源于对多种产品的消费，对高质量产品的消费使家庭的效用水平更高。企业进入了一场模仿竞赛，可以以一定的成本模仿一些全新的产品。抄袭之后，垄断竞争盛行，新的创新竞赛开始。如果一家公司成功地发明了一种更高质量的产品，那么它就可以占领该产品的整个市场；结果表明，该模型在市场规模与增长率之间呈现倒u型关系。大小经济体增长缓慢，中等经济体增长较快。对于专利长度，本研究得到以下结果：专利长度越长，创新水平越高，从而导致更高的增长。当时间偏好率较小时，存在使福利最大化的最优专利长度，即家庭不近视。

This research constructs an endogenous growth model which combines the quality ladder model and the variety expansion model. Households derive their utility from consumption of variety of products and enjoy a higher utility level from consumption of products of higher quality. Firms enter an imitation race and can imitate some brand-new products with a certain cost. After copying, monopolistic competition prevails and a new innovation race begins. If a firm succeeds in inventing a higher quality product, then it can take over the entire market of the product ; however, the existence of variety of products constrains the highest price which the successful firm can charge We show that the present model exhibits an inverted U-shape relationship between the market size and the growth rate. Relatively large and small economies grow slowly while middle sized economies grow fast. As for patent length, this research obtains the following results. The longer the patent length becomes the higher the level of innovation and this leads to higher growth. There exists an optimal patent length which maximizes the welfare when the rate of time preference is small, that is the households are not myopic.