In the Papua New Guinea highlands, the staple food crop of sweet potato sometimes becomes scarce - with no apparent cause. Research over the course of a decade discovered why. When sweet potato was somewhat scarce, usually as a result of too much or too little rain, village men cleared more fertile land for planting and women increased the planting rate. This resulted in a surplus of sweet potato the following year, so villagers then reversed their behaviour. This leads to a second food shortage - with no apparent cause - about two years after the first. Mystery solved!