Is it possible to tie down the origins of the UK's obesity crisis to a single date or set of circumstances - and what does the exercise teach us?

Cheltenham Science Festival last week taught me a new party game: the debate over which date one could plausibly argue was the start of the obesity crisis.

One person argued for 1984 - the date published figures show a rise. Another proposed 1945 as the end of World War II heralded the preconditions for obesity: the pursuit of unrestrained choice, cars replacing bikes and rife consumerism.

Another suggested 1948 when the Co-op opened the first self-service supermarket 30 years after Clarence Saunders in the USA.

One colleague offered a more individual perspective: obesity begins at conception. Taking just one factor, there's evidence that breastfed babies are less likely to be obese in later life.

This was a game but with a serious purpose. The Cheltenham audience went for hard dates and voted for 1984, but not before debating issues such as the role of domestic skills, the arrival of microwaves, why some people get fat and others don't, how obesity illustrates inequalities in health and the growth of soft drinks.

All these dates, data and interpretations were not only plausible, but were rooted in evidence. The Science Festival debate echoed much in the real world, where policy-makers claim they pursue 'evidence-based policy'.

But the true relationship between policy and evidence is not quite so clear cut. Lots of arguments have evidence. At City we teach a different analysis. One can have policy in pursuit of evidence, policies with partial evidence, policies denying evidence, evidence in pursuit of policy and so on. Nor should we forget the default approach - 'eminence-based policy'.

The reality is that this plethora of evidence gets pitched into the policy pot, making policy auditing even

more complex. Politicians want to reduce complexity and relish decisiveness but as the US wit H L Mencken said: "For every complex problem, there is an answer that is clear, simple and wrong".

That's why it is good that, at last, the complexity of climate change is being recognised. It requires massive change, not quick fixes. We've all got to consume less and differently. It's similar for obesity.

If water and electricity companies realise that they've got to encourage less consumption will food trades follow?

How about selling less? Now that is a different business model, but isn't 'choice editing' a move that way?n

Tim Lang is professor of food policy at City University