No one owns a Kindle for very long without becoming obsessed by its social highlighting feature: unless you go into the preferences to turn it off, the glibbest and most epigrammatic sentences in any popular book begin to appear with dotted lines underneath them and the words '19 [or however many] people highlighted this'. Our own Mark Mason has written brilliantly and sympathetically about the consequences. But it is now necessary to admit that he may have missed a trick.
It turns out you may be able to use Kindle highlights to make a rough estimate of how many people are actually reading a book, as opposed to just buying it. The technique was described by Jordan Ellenburg in the Wall Street Journal; I saw it on the reliably interesting academic blog Crooked Timber. He calls it the Hawking Index, after that great unread classic A Brief History of Time:
'Every book's Kindle page lists the five passages most highlighted by readers. If every reader is getting to the end, those highlights could be scattered throughout the length of the book. If nobody has made it past the introduction, the popular highlights will be clustered at the beginning.
'Thus, the Hawking Index (HI): Take the page numbers of a book's five top highlights, average them, and divide by the number of pages in the whole book. The higher the number, the more of the book we're guessing most people are likely to have read. (Disclaimer: This is not remotely scientific and is for entertainment purposes only!)'
On this measure, Donna Tartt's The Goldfinch scores 98.5%, indicating compulsive readability and a highly quotable ending; and Thomas Piketty's Inequality in the Twenty-first Century manages 2.4%, indicating either an exceptionally cogent and pithy introduction or a great many unread copies (or both). A Brief History of Time scores a relatively respectable 6.6%, but as Crooked Timber's Harry Brighouse points out, this is probably a flattering result: the vast bulk of the unread copies will be in good, old-fashioned print.