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.
If we don't reverse the current trend in food prices, we've got until August 2013 before social unrest sweeps the planet, say complexity theorists
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. More