 The wealth of an economy is not a fixed amount from one period to the next, but can be influenced by many factors relevant to economic growth. These include, for example, technological advancement or changes in policy that affect how much people are able to produce with their labour and resources. More wealth can be produced and indeed this has been the overwhelming feature of industrialized countries over the last couple of centuries. The dominant economic view is that wealth is most readily increased in systems where those who are more productive earn greater incomes. This economic view partly inspired the formulation of the Difference Principle. 