From 1891 to 1902, the twenty most frequently borrowed books from the Muncie Public Library in Indiana had a median publication date of 1878. From July 2009 to June 2010, the twenty most frequently borrowed books from UK public libraries had a median publication date of 2009. Among the top 250 borrowed books from UK public libraries in 2009/10, only twelve were published in 2005 or earlier. These data suggest that the time since publication of popularly read books has shortened greatly over the past century.
An increase in the share of adult books among borrowed books accounts for some of the reduction in popular-book publication age. The twenty most frequently borrowed books from the Muncie Public Library are predominately young adults’ books. The twenty most frequently borrowed books in the UK in 2009/10 are predominately adult books. Comparison between adult and children books among most frequently borrowed books in the UK shows that the adult books have more recent publication dates. The eight earliest publication dates among the top-250 most frequently borrowed books in the UK in 2009/10 are all children’s books.
Nonetheless, popular-book age decreased greatly even recognizing the different lifespans of children’s and adults’ books. Among the top twenty most frequently borrowed children’s fiction in the UK in 2009/10, the median publication date is 2007. That implies a median age of under four years, compared to a median age of greater than 12 years for the top twenty most frequently borrowed titles from the Muncie Public Library from 1891 to 1902.
More popularly influential mass media and greater popular communication capabilities are a plausible explanation for the lesser publication age of books most frequently borrowed from public libraries. Greater social influence has been empirically associated with less predictability of social choices and greater popularity among the most popular items. Greater social influence is also plausibility associated with faster changing book-reading interests.
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Sources and data: The Muncie Public Library circulation data is available via Ball State University’s What Middleton Read website. The UK library circulation data comes from the UK’s Public Lending Right. The Guardian has a general overview of the UK data. Here are the most frequently borrowed books for Muncie and the UK (Excel version), augmented and arranged to be directly relevant to the above discussion.
 An obvious limitation is that the earlier data are for only one public library. About 700 public libraries across the U.S. still have library records from the 19th or early 20th century. I hope that other public libraries will follow the example of Muncie Public Library and Ball State University in digitizing historic library records and making them available on the web. Wayne Wiegand has made available records of the book collections of five U.S Midwestern public libraries from 1890 to 1970. Expanding this dataset is also a worthwhile project.
 See Matthew J. Salganik, Peter Sheridan Dodds, and Duncan J. Watts, “Experimental study of inequality and unpredictability in an artificial cultural market” Science, 311, 854-856 (2006).