Britain's booming population: "unalloyed good news" or "full to bursting"?

"A quarter of last year's babies have mothers who were born abroad, up 6% on a decade ago — but, says the ONS, they are not the main reason for the increase in the birth rate."

Polly Toynbee, The Guardian, 9 August 2013

"The ONS figures clearly show that immigration is the main driver of population growth in the UK."

Migration Watch, quoted in The Daily Star, 9 August 2013

As thousands of Brits head for the airport to seek out sunnier climes, the number entering through the arrivals gates was the centre of much attention in today's newspapers.

Yesterday, the Office for National Statistics released its latest population estimates, finding that the increase in the number living in the UK was greater than in any other European country in 2011/12.

Whether or not you saw this as welcome news might well depend upon which newspaper you picked up this morning: while Guardian columnist Polly Toynbee wrote that the figures represented "unalloyed good news", the Daily Star took a rather different line, claiming that Britain is "full to bursting with immigrants".

So what did the ONS actually say?

The ONS noted a 419,900 (0.7%) increase the UK population in the year to 30 June 2012. Over the longer term, Britain has gained nearly 4.6 million people since mid-2001.

Whether immigration is the "main driver" of this growth is trickier to pin down.

What we know from the most recent ONS release is that 39% of this year's increase came directly from net migration (165,600 more people entered the country to live than left it). The remaining 61% of the growth came from what the ONS terms "natural change", which covers a boom in births, as well as improvements in life expectancy.

Will somebody think of the children?!

Of course, as we've seen before, it isn't always easy to entirely separate 'natural change' from immigration, as many immigrants will of course go on to have children of their own, potentially pushing up the birth rate.

In fact, a recent briefing by Oxford University's Migration Observatory pointed to one study which estimated that 65% of the increase in the birth rate was accounted for by babies born to foreign-born mothers.

Even this however won't give the whole picture, as it doesn't account for the effect of emigration (potential parents moving abroad could result in a lower number of births in this country than would have been the case had they stayed).

If we accept these limitations, looking forward, the Migration Observatory forecasts that the combined effects of migration and the increased births to foreign-born mothers will account for two-thirds of the population growth over the next two decades.

Is migration good for the economy?

According to Ms Toynbee in the Guardian, the boost in population is good news for Britain's struggling economy.

She points to a report from the Office of Budget Responsibility which she cites as claiming that without immigration:

"the UK's public sector debt would rise from 74% of GDP today to 187% by the middle years of this century."

As we can see from the chart below - which is taken from the report in question - the size of the national debt would grow to around 180% of GDP by the 2060s in a 'natural change' only scenario, where the only change to the population is driven by births and deaths in the 'native' population. In a 'high migration' scenario, the debt to GDP ratio at this point would be similar to the level it is at today.

However as Jonathan Portes of NIESR has pointed out, these results actually owe more to demographics than anything else, and the OBR hasn't suggested that there is anything unique to the migrant population that accounts for this reduction, and admits that there is mixed evidence on the relative levels of productivity among the migrant and native populations.

Rather, the OBR has assumed that migrant workers go about their business just as the native population would. The fall in the public sector debt that accompanies higher migration is explained by the fact that immigrants tend to arrive in the country as working-age adults, meaning the state doesn't have to subsidise their education, and will often leave the country before they retire, thereby saving the state in retirement benefits.