AGING AND PRODUCTIVITY AMONG ECONOMISTS (来源：www.EnglishCN.com)
Abstract--Economists' productivity over their careers and as measured by publication in leading journals declines very sharply with age. There is no difference by age in the probability that an article submitted to a leading journal will be accepted. Rates of declining productivity are no greater among the very top publishers than among others, and the probability of acceptance is increasingly related to the author's quality rather than the author's age.
It is well known that productivity declines with age in a wide range of activities. Lehman (1953) suggests an early peak in productivity in a variety of scientific and artistic endeavors, and Diamond (1986) documents the pattern for several scholarly pursuits. Levin and Stephan (1992) provide clear evidence that this decline exists even after careful attempts to account for individual and cohort differences. Fair (1994) finds declines in physical ability among elite runners, as does Lydall (1968,pp. 113 passim) in physical abilities of the population generally. In this study we examine productivity declines in our own field. The main new results arise from our use of two different types of information, the equivalent of household and establishment data, to study the stone field over essentially the same period of time. Section I discusses the general results on aging and productivity, whereas section II presents evidence of the importance of heterogeneity.
Using the American Economic Association (AEA) Directory of Members, we identified tenured economics faculty at 17 top research institutions and obtained the years of their Ph.D. degrees. With the citation index of the Journal of Economic Literature we replicated portions of the curricala vitae of each of the 208 economists currently in the economics departments of those institutions who received Ph.D. degrees between 1959 and 1983.
To measure productivity we construct three indexes, combining papers published in refereed journals. Prior research suggests that, at least in terms of salary determination, the returns from nonreferred publications are quite low Sauer (1988), so that we ignore such publications in calculating these measures. I1 weights an article by the journal where it appears based on citations to that journal, using values generated by Laband and Piette (1994). This index distinguishes strongly among journals. For example, the Journal of Political Economy has a weight of 59.1, whereas Economic Inquiry has a weight of 7.9. In constructing I1 we use the weights associated with the decade in
which the articles were published. I2 distinguishes somewhat less among journals by assigning all articles in the nine "core" journals identified by Laband and Piette a value of 1, whereas all other journals are valued at 0.5. Finally, I3 gives all papers a weight of 1. Coauthored articles were given half credit, consistent with Sauer's (1988) findings on the economic returns to coauthorship.
We measure the change in productivity over the life cycle by the percentage change in the number of publications from 9-10 years past the Ph.D. to the periods 14-15 years and then 19-20 years after. For most of the elite economists the base period is equivalent (accounting for publication lags) to the time of tenure, when one might expect that incentives to produce are at a peak. Using two-year publication records at each point reduces the effects of noise in the performance measures. One might argue that still other scientific life-cycle mileposts (e.g., attaining a full professorship) should be accounted for too (and to some extent the 14-15-year point does this). But our main purpose is simply to provide detailed evidence on the relationship to age, and our data are not sufficient to infer the impact of every possible milepost.
Table 1 contains data on productivity loss by Ph.D. vintage measured by each of the three indexes. If we consider I1 and I2, the two indexes that take journal quality into account, the decline appears to be quite substantial. Between years 9-10 and 14-15 elite economists as a group lose 29 to 32% of their output. From years 9-10 to 19-20 they lose 54 to 60%. In other words, productivity losses are on the order of 5 % per year from the time of peak productivity. However, the losses do not appear to accelerate over these 10 years of the economists' work lives. The loss from year 10 to year 20 is approximately twice that from year 10 to year 15.