The latest instalment of our weekly series, in which we use the Centre for Cities’ data tools to crunch some of the numbers on Britain’s cities.
Whenever we write about the demographics of cities, there's an assumption we make so casually that it's easy to forget it's an assumption at all: that everything is down to economics.
When we see that Milton Keynes, London or Cambridge are some of Britain's fastest growing cities we think: “Well, that's obviously because they're boom towns, isn't it? Stands to reason.” Then we pat ourselves on our backs for our perspicacity and go merrily upon our way.
The thing is, we've never actually stress tested the logic of this assumption. Sure, cities like Manchester ballooned rapidly during the industrial revolution, as people moved from rural areas to be near jobs; and you can see that trend at work in the developing world right now.
But all that happened at a time of economic revolution. Does the same necessarily hold true in a developed country – especiallly one where it's so hard to build enough housing?
Well – yes, as it turns out. This graph plots the population change between 2011 and 2013 in 62 British cities, against weekly wages in 2012.
It's not the strongest correlation we've ever seen: some high wage cities like Aldershot and Derby are barely growing at all, while the biggest population growth has happened in relatively low-wage Coventry.
Nor is it hard to think of reasons why cities would buck the trend. Many high wage cities also have high housing costs, making it hard to move into them and limiting their population growth. Some cities double as commuter towns for others nearby (Luton's growth, for example, is probably explained by its proximity to London).
But the correlation is there, nonetheless: even in a developed country with a moribund planning system like Britain, booming cities tend to be growing cities, too.
That doesn’t mean that high wages lead to population growth, of course: correlation is not causation. Even if there is causation, it;’s far from clear which way it runs. Do booming cities attract more people? Or does population growth create economic booms? Maybe, just maybe, it’s both.