In the 1970s the banks in rich countries had a lot of money from oil revenues. They loaned money to poorer countries at low rates of interest. When economic forces caused the banks to raise their interest rates and the commodity prices to fall, the poorer countries could not afford to pay back. The banks extended the loan periods and the debtor countries tried to increase exports in order to earn enough money to pay back the banks. By the end of the 1980s the developing countries were paying three times more money to the banks than they were receiving in aid from the richer countries.