On the Shoulders of Giants: The Growing Impact of Older Articles

In this paper, we examine the evolution of the impact of older scholarly articles. We attempt to answer four questions. First, how often are older articles cited in scholarly papers and how has this changed over time. Second, how does the impact of older articles vary across different fields of scholarship. Third, is the change in the impact of older articles accelerating or slowing down. Fourth, are these trends different for much older articles. To answer these questions, we studied citations from articles published in 1990-2013. We computed the fraction of citations to older articles from articles published each year as the measure of impact for the study. For this study, we considered articles that were published at least 10 years before the citing article as older articles. To explore how changes in citation behavior differ across areas of research, we computed these numbers for 261 subject categories and 9 broad areas of research. Finally, we repeated the computation for two other definitions of older articles, 15 years and older and 20 years and older. There are three major conclusions from our study. First, the impact of older articles has grown substantially over 1990-2013. Our analysis indicates that, in 2013, 36% of citations were to articles that are at least 10 years old and that this fraction has grown 28% since 1990. The fraction of older citations increased over 1990-2013 for 7 out of 9 broad areas of research and 231 out of 261 subject categories. Second, the change over the second half (2002-2013) was significantly larger than that over first half (1990-2001) — the increase in the second half was double the increase in the first half. Third, the trend of a growing impact of older articles also holds for articles that are at least 15 years old and those that are at least 20 years old. In 2013, 21% of citations were to articles ≥ 15 years old with an increase of 30% since 1990 and 13% of citations were to articles ≥ 20 years old with an increase of 36% over the same period. Now that finding and reading relevant older articles is about as easy as finding and reading recently published articles, significant advances aren’t getting lost on the shelves and are influencing work worldwide for years after.