What is the Prebisch-Singer Hypothesis?

-Demand for Primary Products are Income Inelastic. So as Incomes rise, Demand changes quite Little -Demand for Manufactured Products are more Income Elastic. So as Income rises, Demand changes a lot more -As the Price of Manufactured Goods Increases, Nations exporting Primary Products will be Unable to Import Fewer Manufactured Goods for a Given Level of Exports -Thus Relying on Cash Crops and Primary Products are not Long Term