The Prebisch Singer hypothesis is an economic theory developed by Raul Prebisch and Hans Singer. The theory states that the terms of trade between primary goods and manufactured products deteriorate over time. What this means is that countries that export primary goods that do not have the means to manufacture goods to export will lose out in the long-run, as their goods will become relatively cheaper than the manufactured ones. A common explanation for the phenomenon is the observation that the income elasticity of demand for manufactured goods is greater than that for primary products - especially food. Therefore, as incomes rise, the demand for manufactured goods increases more rapidly than demand for primary products.
The worry with this is that the main exporters of primary goods are developing countries. If we take this hypothesis as fact, then that does not bode well for developing countries because it means that any hope of ever industrialising without borrowing large amounts (which is hard with a low credit rating) are slim - as they will not be making much profit on their exports, while being faced with higher costs of imports.