Here are the the top 25 most popular Library of Congress Subject Heading (LCSH) pages on LibraryThing, according to Google Analytics.

I’m guessing this makes someone at the Library of Congress blush:

1. Erotic Stories, American
2. Photography of the nude
3. Erotic fiction
4. Historical fiction
5. Erotic literature
6. Love stories
7. Psychological fiction
8. Fantasy fiction
9. Mystery fiction
10. Erotic art
11. Detective and mystery stories
12. Characters and characteristics in literature
13. Sex instruction for gay men
14. Sexual dominance and submission
15. Humorous stories
16. Symbolism in literature
17. Murder
18. Australia > Social life and customs
19. England > Social life and customs > 18th century
20. Social classes
21. Allusions in literature
22. Humorous fiction
23. Sex instruction
24. Short stories
25. Religion

The explanation involves a paradox. Erotica does so well because LibraryThing is a non-erotic site. The top subjects all win because of search-engine referrals. Google likes a mix of sites, so that erotic searches turn up something besides erotica. (This is particularly true if you have “safe search” enabled.) And LibraryThing has relatively high PageRank (PR), Google’s measure of a web site’s authority. Put these factors together and LibraryThing turns up high for erotic searches. For example, we’re currently Google’s number one site for “gay sex instruction.” Who would’ve thunk it?*

Of course, the “bounce rate” for these pages is astronomical. LibraryThing provides no actual sex instruction, just links to books about it—or rather links to metadata about books on sex instruction. That’s not what the searchers were looking for, and they leave as fast as they arrive.

As a side note, it’s sad to see so many top-level subjects in the list. I hope the bounce rate isn’t too high. Top-level subjects are where LCSH falls apart. Take a subject like “Historical fiction,” which has almost 8,000 works underneath it and no innate relevancy ranking. There can be little doubt–people don’t want to plough through 8,000 links!

*Can we start running ads on just the erotic pages.