I stumbled on this while looking for something else – the Singer-Prebisch thesis. What Singer and Prebisch found (separately and concurrently) is that the terms of trade between primary commodities and manufactures was declining over time. If true, this empirical observation has profound implications for economic development.
Let me explain that in English.
The terms of trade, put simply, is the amount of imports you can “buy” with one unit of exports. In other words, it measures the purchasing power of exports.
If your terms of trade are declining over time, you have to keep producing more and more just to be able to afford the same quantity and value of imports. But commodity production is subject to inelastic supply – it’s extremely difficult to continually ramp up production.