It starts with an apparent mystery. At the end of 2006, food prices across the world started to rise, suddenly and stratospherically. Within a year, the price of wheat had shot up by 80 per cent, maize by 90 per cent, rice by 320 per cent. In a global jolt of hunger, 200 million people–mostly children–couldn’t afford to get food any more, and sank into malnutrition or starvation. There were riots in more than 30 countries, and at least one government was violently overthrown. Then, in spring 2008, prices just as mysteriously fell back to their previous level. Jean Ziegler, the UN Special Rapporteur on the Right to Food, calls it “a silent mass murder”, entirely due to “man-made actions.”

Earlier this year I was in Ethiopia, one of the worst-hit countries, and people there remember the food crisis as if they had been struck by a tsunami. “My children stopped growing,” a woman my age called Abiba Getaneh, told me. “I felt like battery acid had been poured into my stomach as I starved. I took my two daughters out of school and got into debt. If it had gone on much longer, I think my baby would have died.”

Most of the explanations we were given at the time have turned out to be false. It didn’t happen because supply fell: the International Grain Council says global production of wheat actually increased during that period, for example. It isn’t because demand grew either: as Professor Jayati Ghosh of the Centre for Economic Studies in New Delhi has shown, demand actually fell by 3 per cent. Other factors – like the rise of biofuels, and the spike in the oil price – made a contribution, but they aren’t enough on their own to explain such a violent shift.

To understand the biggest cause, you have to plough through some concepts that will make your head ache – but not half as much as they made the poor world’s stomachs ache.

For over a century, farmers in wealthy countries have been able to engage in a process where they protect themselves against risk. Farmer Giles can agree in January to sell his crop to a trader in August at a fixed price. If he has a great summer, he’ll lose some cash, but if there’s a lousy summer or the global price collapses, he’ll do well from the deal. When this process was tightly regulated and only companies with a direct interest in the field could get involved, it worked.

Then, through the 1990s, Goldman Sachs and others lobbied hard and the regulations were abolished. Suddenly, these contracts were turned into “derivatives” that could be bought and sold among traders who had nothing to do with agriculture. A market in “food speculation” was born.

So Farmer Giles still agrees to sell his crop in advance to a trader for £10,000. But now, that contract can be sold on to speculators, who treat the contract itself as an object of potential wealth. Goldman Sachs can buy it and sell it on for £20,000 to Deutsche Bank, who sell it on for £30,000 to Merrill Lynch – and on and on until it seems to bear almost no relationship to Farmer Giles’s crop at all.

If this seems mystifying, it is. John Lanchester, in his superb guide to the world of finance, Whoops! Why Everybody Owes Everyone and No One Can Pay, explains: “Finance, like other forms of human behaviour, underwent a change in the 20th century, a shift equivalent to the emergence of modernism in the arts – a break with common sense, a turn towards self-referentiality and abstraction and notions that couldn’t be explained in workaday English.” Poetry found its break with realism when T S Eliot wrote “The Wasteland”. Finance found its Wasteland moment in the 1970s, when it began to be dominated by complex financial instruments that even the people selling them didn’t fully understand.

So what has this got to do with the bread on Abiba’s plate? Until deregulation, the price for food was set by the forces of supply and demand for food itself. (This was already deeply imperfect: it left a billion people hungry.) But after deregulation, it was no longer just a market in food. It became, at the same time, a market in food contracts based on theoretical future crops–and the speculators drove the price through the roof.