Life Cycle Productivity in Academic Research: Evidence from Cumulative Publication Histories of Academic Economists

Casual observation suggests that the temporal pattern of productivity varies dramatically across individuals, particularly in risky endeavors such as a research career in academia. Some individuals remain productive researchers throughout their career. Others who show early promise become deadwood before their time. Some authors produce papers like a well oiled machine, while others publish at a highly erratic rate. Opposing this is a widely held view that there is a common life-cycle component underlying individual productivity. The aging process itself is thought to lead to the initial rise and eventual decline of creative productivity. In addition, human capital models typically imply that investment in skills will decline with age [7]. Both of these ideas suggest that output will be hump-shaped over the life cycle. Econometric studies of life cycle productivity have addressed this with regressions which are quadratic functions of age or experience [1; 5]. The quadratic conveniently delivers the sought after hump-shaped pattern in productivity. There are conceptual problems with the quadratic however. It implies a symmetric output path centered on its peak, which is inappropriate if productivity peaks early in the career but remains significant thereafter. In addition, data on publishing productivity is heavily affected by the "zeros problem:" the most common number of publications for an active professor in any given year is zero. This requires an estimation procedure designed to accommodate a high probability mass at zero. Levin and Stephan [5] address this problem with the tobit regression. The tobit is a truncated regression model, where in this context publications are distributed as a truncated normal random variable if a vector combination of attributes and coefficients exceeds a threshold level, and are zero other-