Julian Simon, The Great Breakthrough and Its Cause, puts the trigger of the industrial revolution on the total population. After reading it, I think that **communicating population** size might be a better criterion, that is not just the sheer number of people in an area, but the number of interacting people (non-serfs). Before someone protests the examples of China and India, he points out that until about 1400 China was leading, and goes on to state (my terms not his) that population size is necessary, but may not be sufficient without other factors.