The first Amazon review of my new book showed superb taste: it gave me the top rating of five stars. Naturally, my ego is tickled by the fact that the reviewer - his name is Alistair Kelman, and he's introduced himself to me online and attended a couple of my talks - so wisely divined the book's boundless excellence. But I am also an economist, and so interested in what this might do to my sales figures - and, therefore, my income.

We know that online reviews make a difference but this isn't totally straightforward to establish. An excellent book (ahem) might win both a large readership and positive reviews; it would seem impossible to prove that the reviews helped boost the sales figures. But two economists, Judith Chevalier of Yale and Dina Mayzlin of the University of Southern California, observed that different websites host different reviews. Observing sales ranks and reviews on Amazon.com and its rival BN.com, Chevalier and Mayzlin concluded that reviews had a substantial impact on sales - with negative reviews being taken particularly seriously.

And yet any mainstream book will accumulate several reviews - perhaps dozens. Doesn't that suggest that Mr Kelman's undoubted discernment is almost irrelevant to my book's prospects? Perhaps not: an initial positive review may encourage others to be positive too - or stir up some disagreement. Now this, too, also seems hard to figure out. One good review will often be followed by other good reviews. Is this because the reviewers are influencing each other, or because they all see the same quality in the book? Is everyone reading Fifty Shades of Grey because it approaches the platonic ideal of soft porn? Or because, well, everyone's reading Fifty Shades of Grey?