How long is the perfect book?

British novelist E.M. Forster once complained that long books “are usually overpraised” because “the reader wishes to convince others and himself that he has not wasted his time.” To test his theory we collected reader ratings for 737 books tagged as “classic literature” on Goodreads.com, a review aggregator with 80m members. The bias towards chunky tomes was substantial. Slim volumes of 100 to 200 pages scored only 3.87 out of 5, whereas those over 1,000 pages scored 4.19. Longer is better, say the readers.

The phenomenon that Forster describes, akin to literary “Stockholm syndrome”, is only one possible explanation. Another is survivor bias, since those who like a book enough to finish it may be more inclined to leave a review. A third is cumulative value: a handful of the books longer than 1,000 pages are anthologies, which tend to be judged on their overall worth rather than the quality of each individual piece. Shakespeare claimed that brevity is the soul of wit, but his hefty collected works score 4.49, compared with an average of just 3.8 for his individual plays.

The data show other reading biases, too. Reviewers favour modern tales over old ones, with the exception of some ancient texts that continue to delight, such as Sun Tzu’s “The Art of War”, “Tales from the Arabian Nights” and Marcus Aurelius’s “Meditations”. Most loathe books that they were taught at school but love ones they discovered as children and teenagers: J.K. Rowling and J.R.R. Tolkien dominate the very top of the rankings. But Goodreads users are hardly a bunch of anti-intellectual orcs and trolls: Russian literature is among the mostly highly rated genres. After months of ploughing through “Anna Karenina”, or “Crime and Punishment”, a good rating is as much a reward for the reader as the writer.