Because it is now easier to locate older articles, researchers are citing those works more frequently, says an analysis by Google researchers posted at arXiv.
Google's Alex Verstak and his colleagues examined the age of citations referenced in articles published between 1990 and 2013. Any article cited that was 10 or more years older than the one under examination was considered to be 'older.'
As they report in their paper, the Google researchers say that the fraction of older articles cited has increased. Some 36 percent of citations in 2013 were to articles that were at least 10 years old, an increase of 28 percent since 1990. Additionally, they found that this move toward citing older articles was more pronounced between 2002 and 2013 than between 1990 and 2001.
"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," Verstak and his team say.
While the University of Montreal's Vincent Larivière has observed the same trend as the Google team, he tells ScienceInsider that he disagrees on the cause, adding that he's found the trend to predate the digitization of older papers.
Instead, he points to the rate of growth in science as the force behind the trend. "In periods of exponential growth of science, the [average] age of existing literature, and thus of cited — or citable — literature, is always younger and younger," Larivière says.
After World War II, the age of papers cited was young because science was undergoing an expansion, ScienceInsider says. Now, though, Larivière says "science is growing at a lower rate than it used to, and this affects the age of what is cited."
Alternatively, ScienceInsider offers a "darker explanation" — that recent papers are of low quality and researchers don't find them worthy of citing.