Research complexity increases with scientists’ academic age: Evidence from library and information science

•Complexity is adopted as a novel perspective to characterize scientists’ research portfolios.•An econometric approach is employed to measure the research portfolio complexity (RPC) by modeling a heterogeneous author-topic bipartite network.•Scientists of similar productivity and impact may differ in RPC.•Senior scientists have higher RPC than their junior colleagues, even after removing the accumulated advantages.•Scientists with the highest RPC have distinct dynamic patterns of RPC. With the continued aging of the scientific workforce, the impact of this trend on scientists’ research performance has attracted increasing attention. The literature has predominantly focused on the productivity, impact, and collaboration pattern of scientists of different ages. A research gap is found in investigating the differences in the research topics studied by junior and senior scientists. This study focuses on the complexity of a scientist's research portfolio (RPC). Based on the concept of economic complexity, RPC was measured to characterize the capability of scientists to study complex research topics. An economic algorithm was adopted to estimate RPC on heterogeneous author-topic bipartite networks using bibliographic data from the field of Library and Information Science between 1971 and 2020. Through comparisons among scientist groups, RPC shows promise in distinguishing outstanding scientists from peers who have similar values of other indicators (e.g., citations and H-index). The change in RPC was further probed across scientists’ careers and an increasing trend with academic age was found, even after removing the accumulated advantages of senior scientists. Moreover, top-ranked scientists distinguish themselves from their peers by a higher RPC in the first year and a greater growth rate during their careers. While many researchers have their highest RPC in the first year, most top-ranked scientists reach their peak RPC later in their careers. The results provide helpful references for studies on the aging effect in academia.