Last week in his regular column for the Evening Standard, the Mayor of London addressed the issue of population growth and the shortage of school places in London.
"London is booming — with a population that has risen by 600,000 since I became Mayor — and that amazing statistic is not principally a function of immigration, as it happens, but of the simple rate of live births against deaths."
But can this be accurate? After all, we know from the Census that for the first time in 2011 the majority of London's population wasn't "White British".
When Boris Johnson was elected, 2.5 million (or 33%) of the capital's residents were non-UK born, including the Mayor himself who was born in the United States. By 2011 this had grown by around 500,000 to 3 million (37%).
These estimates warrant a note of caution. The 2011 Census shed light on sub-national population estimates from mid-2002 to mid-2010. Local authorities in London were subject to the largest revisions. The ONS notes that the largest percentage revision was for the City of London, with a downward revision of 38% across the decade. The 2008 figures were not subject to this revision, however, so direct comparisons cannot be precise.
London's UK-born population declined by 1% (around 50,000 residents) in the ten years up to 2011 according to Census data analysed by research institute Migration Observatory. Almost, if not all the growth of the London population in that decade can be accounted for by increase in the number of residents that had been born outside of the UK.
In his column, Mr Johnson explains the growth in residents as a result of "the simple rate of live births against deaths". He adds: "Londoners are living longer, and producing more children, and we will need an extra 118,000 school places by 2016."
Births and the birth rate in London have indeed risen since Boris became Mayor. In 2008 London saw 127,651 live births, and that figure went up by 5,000 in just four years. The long-term trend shows an unambiguous rise in the number of London live births. Londoners can increasingly expect to live longer too.
Similarly, deaths and the mortality rate are falling, both since Boris became Mayor and on a clear longer term trend. There were 46,685 deaths in London in 2001, down from 50,513 in 2008.
But "the simple rate of live births against deaths" does not tell us about the overall population of London because it ignores the large numbers of people who move out of London within the UK — about 40,000 net in 2011.
Needless to say, the idea that "Londoners are living longer, and producing more children" isn't at odds with the notion that foreign-born London residents are also having children. Who exactly, is "producing more children"? UK-born or foreign-born residents?
A Parliamentary Question posed by Nicholas Soames MP unearthed a remarkable statistic on the number of children with foreign born parents: for 64% of London births in 2011, at least one parent is foreign-born. This is something which we also looked into last year.
If we look at the breakdown of parental origins - as provided to MPs - we see that 44% of live births are in families where both parents are foreign born. In one fifth of all cases, only one parent was foreign-born. However, these children aren't all necessarily a product of immigration, simply because "foreign-born" doesn't always equate with migrant.
As far as the Office for National Statistics is concerned, the Mayor of London is classed as foreign-born and his children are second-generation migrants. Though both his parents were British, official statistics don't make a distinction between migrants and natives, but rather focus on whether individuals were born in the UK or abroad. This goes to show that immigration is a concept that can, at times, be hard to pin down.
The figures show that London's booming population has a great deal to do with people born outside the UK moving to the capital. The Mayor is right to observe that birth rates are up too, but this is turn owes something to immigration.
Image courtesy of the Greater London Authority (GLA)