Freakonomics, which began life as a book, has turned into a brand. There is the Freakonomics movie, the Freakonomics radio show, the Freakonomics blog. You can hire the Freakonomics guys for business consultancy, for conferences, for after-dinner speaking (maybe for weddings and bar mitzvahs too). As with many brands that expand too quickly, what all this frenetic activity has done is dilute what made the original so successful. The first book worked brilliantly because it was based on a sound economic principle: the division of labour. 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. Stephen Dubner was a jobbing magazine writer with a knack for telling a story. When Dubner met Levitt, he realised he could turn his research into a series of gripping stories. That's how it worked. Dubner was the messenger. Levitt was the message.

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. Dubner presents the radio show, curates the blog and promotes the Freakonomics name at every opportunity. Levitt, who still has a full-time academic career, appears more rarely, and hardly ever to talk about his own latest findings. 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. This is fine when there are Levitt's sparkling alternative explanations to back it up. 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. Especially when the unsayable too often turns out to be surprisingly banal. 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. This is hardly news. And Dubner, though he can do smug, is not even an economist. But it's what he and Levitt are now peddling.

The title of their new book, the third in the series, is evidence of the falling off. Gone is the economics part of the equation. Now it's simply about thinking like a Freak. But without the hardcore economics, thinking like a Freak just means being willing to think outside the box, and there are plenty of people doing that already. Too much of this book is taken up with routine problem-solving advice and over-familiar stories repackaged by Dubner as though they were revelations. "Never, ever think people will do something just because it's the 'right' thing to do," is one of their six guidelines for thinking like a Freak, which is both unoriginal and untrue (it's the "never, ever" that's wrong, since every now and then that's exactly what you should think).

Half a chapter is devoted to comparing the stories of Solomon, king of the Israelites, and David Lee Roth, lead singer of the rock band Van Halen. Dubner revels in the apparent incongruity of the comparison (both men were Jewish – who could have guessed?): he assumes we know one story (about cutting the baby in half) but not the other – about Roth insisting in his concert rider that all the brown M&Ms be removed from a bowl of the sweets in his dressing room. Roth's demand, which might be seen as crazy prima donna pickiness, turns out to be a species of Solomonic wisdom, because by burying the requirement deep in a list of important safety checks, he could know that if brown M&Ms appeared in his dressing room then the venue was likely to be unsafe. It's a nice tale, but it's been doing the rounds for years, to the point where Roth is almost better known for the M&Ms than for his raucous singing and insatiable womanising. It would have been more of a surprise to discover that he was really a dumb rock star after all.

Where the Freakonomics formula still works is when Dubner picks up on some interesting recent research and weaves a compelling story around it. There are two great examples in this book, though strikingly in neither case is the research by Levitt. One concerns Nigerian email scams. We've all had them: the ones that tell you a huge sum of money needs to be transferred out of Nigeria and that you will get a big chunk of it if you allow your bank account to be used as a temporary deposit point. All you have to do is supply your bank details. Who on earth falls for that any more, especially now that the mere mention of Nigeria is enough to signal to almost everyone that it is a con? Work by the computer scientist Cormac Herley shows why the scammers still insist on specifying Nigeria. Sending out millions of emails is more or less costless, so having millions of people ignore them doesn't really matter. What would cost the criminals is spending time and money setting up a fake transfer with people who twig halfway through that it's a trick and get cold feet. By choosing Nigeria they are importing a gullibility-testing device into the original pitch to discover the vanishingly small number of people who still don't know. Anyone who responds has already signalled that they are completely in the dark. It's a neatly efficient way of ruling out false positives and tracing the few clueless needles in the haystack of the worldly wise.

The other story concerns slavery and heart disease. The economist Roland Fryer wanted to know why African-Americans were so prone to fatal heart problems relative to the rest of the population. One day he saw an old print called "An Englishman Tastes the Sweat of an African", which showed a slave-trader licking a slave. By a circuitous route he discovered that this might have been a crude test for the likelihood a slave would survive transportation across the Atlantic. A high level of salt in sweat would indicate a propensity to water retention, making a slave less likely to die of dehydration during the hideous crossing. But salt sensitivity is also a strong indicator for hypertension and it is a highly hereditable trait. So slaves who were more likely to survive the passage to America were also more likely to pass on heart disease to their descendants. This is a genuinely unconventional and deeply uncomfortable explanation, in the spirit of the first Freakonomics book, whose most famous story explained falling crime rates through the legalisation of abortion (potential criminals were the ones more likely to be aborted). As with that explanation, Fryer's is highly controversial and it has been widely challenged. But it certainly makes you think.

Unfortunately, in this book that is the exception. The bulk of it is taken up with Dubner overstating the significance of more routine findings and building up his own part in the process. As with a lot of self-help and business consultancy, too much of what's true here is old news and too much of what's new isn't actually true. The most excruciating example comes at the beginning, when we are told what happened when the Freakonomics duo were called in to advise David Cameron shortly before he became prime minister. "All right," boomed Cameron as he strode purposefully into the room, "where are the clever people?" The clever people then proceeded to give him a lesson in the problems of socialised healthcare. The trouble with the NHS, they explained, is that offering any product or service free at the point of consumption inevitably skews the incentives of both providers and consumers. In the Freakonomics spirit of speaking truth to power, Levitt and Dubner told Cameron a story. "What if every Briton were also entitled to a free, unlimited supply of transportation? That is, what if everyone were allowed to go down to the car dealership whenever they wanted and pick out any new model free of charge, and drive it home?"

At this point, they tell us, Cameron, who had been all bonhomie, became much frostier. "The smile did not leave David Cameron's face, but it did leave his eyes." They take this as evidence that even intelligent politicians don't like hearing uncomfortable truths that challenge positions to which they are committed. But it seems more likely that Cameron, who is indeed an intelligent politician, noticed they were talking nonsense. After all, it's a ridiculous analogy. People don't go to the NHS and "pick out" their treatment. They are in the hands of doctors and other healthcare professionals who collectively try to find the best treatments for them, within limits. Healthcare is nothing like transportation. If it were, the NHS, whatever future problems it might be facing, could hardly have survived so long (and performed more efficiently than the rival US system, where many patients really are "picking out" their preferred treatments). Only two economists (or rather one-and-a-half economists) could be so arrogant and so ignorant as to think that this was how to talk to a future British prime minister about healthcare. I imagine that what Cameron was really thinking was: if these are the clever people, spare me from the stupid ones.