15 signs that J.K. Rowling wrote the book you're reading

The British literary world was shocked last week to discover that Robert Galbraith, the first-time author of the mystery novel "The Cuckoo's Calling," was really none other than Harry Potter creator J.K. Rowling writing under a nom deguerre.

Rowling's alter ego was originally exposed by an ill-advised tweet from a woman who learned the secret from a partner at the firm that represents the author. To investigate the claim, the Sunday Times of London consulted two experts in computational linguistics who analyzed the text of "Cuckoo's Calling" and compared it to other novels by Rowling and other authors. Both reported compelling evidence that the claim was true, based on stylistic fingerprints that showed up in both Rowling's and Galbraith's work.

This sort of literary wizardry is known as "stylometrics" — a discipline notably absent from the Hogwarts curriculum — and shows up everywhere from high-profile cases like the outing of Joe Klein as the author of "Primary Colors" to legal battles that hinge on authorship disputes. The discipline tends to rely on broad statistical analyses of word usage and sentence construction. But one aspect of stylometrics is rather easy to demonstrate: the use of "rare pairs," or idiosyncratic two-word phrases that an author favors, probably unconsciously, far more than comparable writers.

In fact, a Yahoo News analysis of Rowling's work alongside several dozen comparable texts by other authors has found 15 phrases that betray Rowling as an author.

Take, for example, the vernacular contraction "d'you think":

You can view all 15 Rowling literary signatures on the Yahoo Signal Tumblr.

While there will obviously be similarities in names common to books of the same series — it's no surprise that a computer can notice that many of Rowling's books mention the phrase "Harry Potter" or "Lord Volde" — er, "You Know Who" — the phrases it found in common between all five the books I analyzed bear no significance to the genre. If you find yourself in a Rowling novel, for example, you can expect to have doors swinging shut behind you at every turn:

You may also find yourself panting slightly:

By themselves, these similarities may not seem hugely compelling; Rowling is far from the first author to borrow a canine verb and then qualify it. But these phrases show up far more commonly in Rowling books than in comparable novels. Here are 12 more phrases that bear a strong Rowling correlation:

• bloodshot eyes

• dimly lit

• half-moon

• halfway along

• inside pocket

• still clutching

• stone steps

• stopped talking

• strode back

• tightly around

• voice rang

• within seconds

The presence of any one of these phrases is not a giveaway. Each one merely raises the odds that a text of unknown authorship belongs to the "Harry Potter" author, the same way genetic tests poll thousands of single-nucleotide polymorphisms to gather a probabilistic sense for whether a specimen is of a given origin.

It's unlikely that anyone would have noticed the subtle similarities in style between "Cuckoo's Calling" and the "Harry Potter" books without a little help from a gossiping social media denizen — a "Rita Tweeter," if you will. But these methods are surprisingly accurate given enough text to analyze. It is no indictment of Rowling's caliber as an author. Even the most inventive authors have a few swinging doors in their arsenals.

Methodology:

To identify these phrases, I gathered the text of four Rowling novels: The final two "Harry Potter" books, Rowling's 2012 novel "The Casual Vacancy" and the Galbraith novel that she has since admitted to writing. I then gathered 36 other books as a control group, including several other British fantasy and mystery novels — four by John le Carré and two by Philip Pullman, for example — as well as a fantasy novel by Ursula K. Le Guin, the final "Hunger Games" book, some diverse books from the public domain and the embarrassing contents of my Kindle.

I then used the Natural Language Toolkit library for Python to identify bigrams — two-word phrases — that appear in at least three of four Rowling books and in no more than four of the remaining 36 books. Finally, I tested for the presence of these phrases in a fifth Rowling book as a control against coincidence. In nearly every case, phrases identified as being common in four Rowling books also cropped up in the fifth.