Adam Smith, Wages reflect the vigour of an economy

The difference between the money price of labour in China and in Europe is still greater than that between the money price of subsistence; because the real recompense of labour is higher in Europe than in China, the greater part of Europe being in an improving state, while China seems to be standing still.

The money price of labour is lower in Scotland than in England because the real recompense of labour is much lower; Scotland, though advancing to greater wealth, advancing much more slowly than England. The frequency of emigration from Scotland, and the rarity of it from England, sufficiently prove that the demand for labour is very different in the two countries.

The proportion between the real recompense of labour in different countries, it must be remembered, is naturally regulated not by their actual wealth or poverty, but by their advancing, stationary, or declining condition.