What’s the number one reason we riot? The plausible, justifiable motivations of trampled-upon humanfolk to fight back are many—poverty, oppression, disenfranchisement, etc—but the big one is more primal than any of the above. It’s hunger, plain and simple. If there’s a single factor that reliably sparks social unrest, it’s food becoming too scarce or too expensive. So argues a group of complex systems theorists in Cambridge, and it makes sense.

In a 2011 paper, researchers at the Complex Systems Institute unveiled a model that accurately explained why the waves of unrest that swept the world in 2008 and 2011 crashed when they did. The number one determinant was soaring food prices. Their model identified a precise threshold for global food prices that, if breached, would lead to worldwide unrest.

The MIT Technology Review explains how CSI’s model works: “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.” Plot the data, and it looks like this:

Pretty simple. Black dots are the food prices, red lines are the riots. In other words, whenever the UN’s food price index, which measures the monthly change in the price of a basket of food commodities, climbs above 210, the conditions ripen for social unrest around the world. CSI doesn’t claim that any breach of 210 immediately leads to riots, obviously; just that the probability that riots will erupt grows much greater. For billions of people around the world, food comprises up to 80% of routine expenses (for rich-world people like you and I, it’s like 15%). When prices jump, people can’t afford anything else; or even food itself. And if you can’t eat—or worse, your family can’t eat—you fight.