Gone is the economics part of the equation. The title of their new book, the third in the series, is evidence of the falling off. But it's what he and Levitt are now peddling. And Dubner, though he can do smug, is not even an economist. Smug economists everywhere love to tell you that people are frequently more self-interested than they appear and that what they say may not be the best guide to what they want. Especially when the unsayable too often turns out to be surprisingly banal. But when it's just two middle-aged men riffing about how appearances can be deceptive and it sometimes takes an economist to say the unsayable, the effect is grating. This is fine when there are Levitt's sparkling alternative explanations to back it up. More often he is wheeled out to display what both men take to be the essence of the Freakonomics approach, which is a straight-talking refusal to kowtow to conventional pieties. Levitt, who still has a full-time academic career, appears more rarely, and hardly ever to talk about his own latest findings. Dubner presents the radio show, curates the blog and promotes the Freakonomics name at every opportunity. Now there is too little of Levitt and far too much of Dubner, who you suspect is the driving force behind the relentless spread of the brand. When Dubner met Levitt, he realised he could turn his research into a series of gripping stories. Stephen Dubner was a jobbing magazine writer with a knack for telling a story. Steven Levitt was a prize-winning economist whose quirky but revelatory approach to everyday problems had made him famous in academia but little-known outside. The first book worked brilliantly because it was based on a sound economic principle: the division of labour. As with many brands that expand too quickly, what all this frenetic activity has done is dilute what made the original so successful.

You can hire the Freakonomics guys for business consultancy, for conferences, for after-dinner speaking (maybe for weddings and bar mitzvahs too).

There is the Freakonomics movie, the Freakonomics radio show, the Freakonomics blog. Freakonomics, which began life as a book, has turned into a brand.

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