In Think Like a Freak, the third and possibly final instalment of the Freakonomics phenomenon, economist Steven Levitt and journalist Stephen Dubner set out to help us master the economic manner of thinking made famous in their previous best-sellers. They provide a summary of this intellectual toolkit early on: incentives are the cornerstone of modern life; knowing what to measure and how to measure it can make a complicated world less so; the conventional wisdom is often wrong; and correlation does not equal causality. In short, get data, analyse them properly, and the world will be a better place. The problem, apparently, is that we still find it hard to admit that we don¡¯t know in the first place.

Levitt and Dubner campaign against received wisdom and prejudices of all kinds, urging us to abandon the faulty ¡°moral compass¡± that convinces us we know right from wrong. We must trust the data and the facts. It is an Enlightenment manifesto offered in straightforward, appealing terms, and anyone schooled in the liberal, empirical Western tradition will feel at home.

The life lessons that the duo offer are illuminating. I was genuinely interested to learn how to become a champion hot-dog eater, to hear precisely why singer David Lee Roth¡¯s famous demand for M&Ms but ¡°absolutely no brown ones¡± makes him a health-and-safety nerd, not a rock¡¯n¡¯roll diva, and why, when writing Superfreakonomics, Levitt and Dubner were at such pains to give away the ¡°secret¡± of their anti-terrorist algorithm. I was delighted, as someone who has harboured a lifelong antipathy to competitive running, to discover that signing up for a marathon is one of the top three life decisions that make people much less happy. It¡¯s all good fun, and the book is worth reading for its chapter titles alone.

But there are moments when the authors should have heeded their own wisdom and admitted that they don¡¯t know either. One of my favourite Think Like a Freak vignettes is an analysis of the medieval trial by ordeal, where it seemed that clerics tampered with the fire to spare the victims¡¯ agonies. Levitt and Dubner offer a ¡°game theory¡± argument to explain why the scalding water and red-hot bars left so many unscathed. It turns out that the threat of ordeal created what modern economics would call a ¡°separating equilibrium¡±: the guilty cut their losses and owned up, while the innocent hoped for God¡¯s intervention but found it more often at the hands of merciful clerics.

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So far, so good, but there¡¯s a typically freaky legerdemain at work. Remember that the economic method uses ¡°as if¡± assumptions to make predictions, so the real argument is: ¡°If we assume that a medieval man thought like a 21st-century game-theorist, can we predict the outcomes of medieval ordeals?¡± Yes? Then we can say that game theory can predict the outcomes of medieval ordeals, and nothing more. To claim, as do Levitt and Dubner, that medieval man thought in game-theory terms is an example of the ¡°formalist¡± fallacy often critiqued by historians of economics, which supposes transcendental, ahistorical forms of economic reasoning.

Nonetheless, Think Like a Freak has an endearing quality. The bombast of Freakonomics¡¯ buccaneer economist, shining a light where no one else dares to look, is gone. In its place is the self-deprecation of the academic dad outsmarted by a three-year-old. Levitt and Dubner are genial, concerned citizens, anxious to help. When your overbearing boss demands an answer right now, they tell us it¡¯s fine to ¡°be crazy enough to admit you don¡¯t know¡±. In a ¡°quitter never wins, winner never quits¡± culture, their quiet advice that giving up is sometimes the right course of action is humanely uplifting.

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There are even moments when the famous ¡°economic method¡± softens a little. Levitt and Dubner talk about decisions being ¡°framed¡± as collaborative, authoritarian or competitive; in other words, embedded in a social context. There¡¯s a discussion of perverse incentives, and the unintended consequences of badly designed markets. I¡¯m not one for disciplinary boundaries, but some of this is starting to sound very sociological. And when they say that economics is a form of narrative, informed by facts and data, they echo heterodox ¡°romantic¡± economists such as Deirdre McCloskey. It¡¯s just another small step towards the terrible realisation that facts are narratives too, and data carry their own moral compasses. By then, the Freakonomists will have tumbled into the wishing well of post-modernism, and it really will be time to quit.