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Thegrowth in the number of people working in research and development might seemto contradict this picture of a less inventive economy: the share of theAmerican economy given over to R D has expanded by a third since 1975, toalmost 3%. But Pierre Azoulay of MIT and Benjamin Jones of NorthwesternUniversity find that, though there are more people in research, they are doingless good. They reckon that in 1950 an average R D worker in Americacontributed almost seven times more to total factor productivityessentially,the contribution of technology and innovation to growththat an R D worker in 2000 did. One factor in this may be the burden of knowledge: as ideas accumulate ittakes ever longer for new thinkers to catch up with the frontier of theirscientific or technical speciality. Mr Jones says that, from 1985 to 1997alone, the typical age at first innovation rose by about one year. 从事研究开发工作的人数的增长似乎同这种缺少发明经济的图景互相矛盾：美国经济拨付给研究开发的份额自1975年以来增长了1/3，接近3%。但是，麻省理工学院的皮埃尔阿祖莱和西北大学的本杰明琼斯发现，尽管从事研究的人数增加了，但是他们的工作成就不如以前。他们推测，在1950年时，美国人均研发人员对全要素生产率从根本上来说，就是技术与创新对增长的贡献做出的贡献几乎是2000年时的的7倍。这其中的一个因素可能是知识的负担：随着知识的累计，后人需要更长的时间才能追赶上他们科研专业领域的前沿。琼斯认为，仅在1985年-1997年间，有代表性的首次发明年龄提高了将近一岁。 | 
