Between World War 2, and the mid 1980s, London's population did something surprising: it fell.
London grew rapidly in the 19th and early 20th centuries until, by 1939, the population of the area now covered by Greater London stood at around 8.6m. Then the Luftwaffe arrived, and for the next four and a bit decades, thanks to war, suburbanisation and the creation of the New Towns, it fell by nearly a quarter, bottoming out at 6.6m at around the time of the 1981 census.
Since then, it's started to grow again, overtaking its previous peak in January 2015. Today, there are more people in London than there have ever been before. Oh happy day.
In some ways, this description is a bit over-simplistic, because it's imposing a modern definition of London on a time before it existed. Greater London didn't come into being until 1965; so looking at population figures for that area in 1939 is a bit like looking at the entire commuter zone now. If we were to look at the entire metro area, it's possible London's population never fell at all.
The different trajectories of inner and outer London’s populations. Source: CityMetric/census data.
At any rate, the point is that, from early in the 20th century, London's workforce were increasingly living at a distance from the centre of the city and commuting in. One odd side effect of this is that many of its inner boroughs actually have smaller populations – in some cases, much smaller – than they did in the late 19th or early 20th centuries. In 1901, what is now Tower Hamlets was home to 597,000 people. By 1981, it was just 142,000, and even by 2015 it was just 297,000.
To get a sense of when the different boroughs peaked, in 2010, the urban planning researcher Steve Chambers made a map. It showed the year at which each borough's population historically peaked, before dropping back again.
One quick note on this. One is that the population of the City of London was higher in 1881 than at any year since – but it was even higher in 1871, and higher still in each decade before that, going all the way back to when recrods began in 1821. It's entirely possible it was at its most densely populated before anyone bothered to count.
Anyway. As Chambers wrote in a blogpost at the time,
the population peaks for each borough tell a story in themselves of people moving outwards, deserting the inner core and, come the 1960s and 1970s, leaving London altogether.
Those figures obviously predate the 2011 census, though, and the world has changed asince then. So here's another map.
The green/blue boroughs are those which still have fewer residents than they once did; once again, the year represents the census at which their population peaked.
The pink/red boroughs are those which have overtaken their previous peak. There, the year represents the first census (or, for 2015, mid-year estimate) at which they over-took that peak.
What you can see at a glance is that outer London is now as heavily populated as it's ever been. But much of the inner city is still empty compared to how it once was. In all, there are still 14 boroughs, plus the City of London, where the population has yet to return to its former peak.
Editor's note: This story was heavily corrected at 11am on 21 March to reflect the fact we'd muffed up the data. Sorry about that. The story remains unchanged however.