When as a crime writer I find myself in front of live audiences, one of the first things I am always asked is why crime fiction is so popular. It's an interesting question, not least because the very people asking it — fans of the genre — are the ones who, in theory, ought to be able to answer it. After all, they buy and voraciously read the stuff. Yet when you put the question back to them they never can. Or at least not satisfactorily.