What causes riots? That's not a question you would expect to have a simple answer.

But today, Marco Lagi and buddies at the New England Complex Systems Institute in Cambridge, say they've found a single factor that seems to trigger riots around the world.

This single factor is the price of food. Lagi and co say that when it rises above a certain threshold, social unrest sweeps the planet.

The evidence comes from two sources. The first is data gathered by the United Nations that plots the price of food against time, the so-called food price index of the Food and Agriculture Organisation of the UN. The second is the date of riots around the world, whatever their cause. Both these sources are plotted on the same graph above.

This clearly seems to show that when the food price index rises above a certain threshold, the result is trouble around the world.

This isn't rocket science. It stands to reason that people become desperate when food is unobtainable. It's often said that any society is three square meals from anarchy.

But what's interesting about this analysis is that Lagi and co say that high food prices don't necessarily trigger riots themselves, they simply create the conditions in which social unrest can flourish. "These observations are consistent with a hypothesis that high global food prices are a precipitating condition for social unrest," say Lagi and co.

In other words, high food prices lead to a kind of tipping point when almost anything can trigger a riot, like a lighted match in a dry forest.

On 13 December last year, the group wrote to the US government pointing out that global food prices were about to cross the threshold they had identified. Four days later, Mohamed Bouazizi set himself on fire in Tunisia in protest at government policies, an event that triggered a wave of social unrest that continues to spread throughout the middle east today.

That leads to an obvious thought. If high food prices condition the world for social unrest, then reducing the prices should stabilise the planet.

But what can be done to reverse the increases. Lagi and co say that two main factors have driven the increase in the food price index. The first is traders speculating on the price of food, a problem that has been exacerbated in recent years by the deregulation of the commodities markets and the removal of trading limits for buyers and sellers.

The second is the conversion of corn into ethanol, a practice directly encouraged by subsidies.

Those are both factors that the western world and the US in particular could change.

Today, the food price index remains above the threshold but the long term trend is still below. But it is rising. Lagi and co say that if the trend continues, the index is likely to cross the threshold in August 2013.

If their model has the predictive power they suggest, when that happens, the world will become a tinderbox waiting for a match.

Ref: arxiv.org/abs/1108.2455: The Food Crises and Political Instability in North Africa and the Middle East