Will Britain’s population reach 70million soon? According to the Balanced Migration group, led by MPs Frank Field and Nicholas Soames, we’ll be hitting that figure some time around 2030, by which time we’ll all be so confined in our crowded little country that the Government could be done for cruelty.

The population increase is down – entirely – to immigration. On Question Time last month Jack Straw answered a question on this subject by pointing out that the Government could hardly control people’s fertility or stop them having children (oh, but I bet they wish they could). He was either being dishonest or stupid– Britain’s fertility rate is around 1.7 children per women, way, way below replacement rate, so that in a migration-free world the population would soon start to tumble once the first of the baby boomers started booking their one-way tickets to Switzerland.

On their website the Balanced Migration group refer to Britain facing a population increase equal to seven Birminghams, the city that is usually used as a unit of population increase these days (the equivalent to the “Wales”, which is a measurement of land space). However, no new Birminghams will be needed for the migrants themselves, of course, but for the natives of that city and other big urban areas who leave as they become increasingly foreign to them – indeed, Birmingham will become the second British city to have a non-white majority way before the UK hits 70 million.

Seven Birminghams also equals one London, a city once seen as a sink that sucked in people from around the country, but which now spits them out at a far greater rate. Over the last four years, on average 70,000 more people have left London for the rest of Britain than have entered, a vote of no confidence in the multi-cultural society if ever there was one. Because however much middle class parents and journalists like to dress this issue up simply as a matter of better schools, safety and space, every single problem ex-Londoners identify with living in the metropolis is heavily aggravated by immigration, and the problems with living in what sociologist Robert Putnam identified as low-trust, socially isolated, multi-ethnic areas.

It is not just about numbers, although south-east England’s high density is certainly one reason why our attempt to mimic America’s melting pot has failed. Were Britain’s population increase down entirely to a high birth rate, it would not have the same effect on people’s quality of life – in fact, it would arguably have less of a social cost than if 400,000 Brits left every year and 400,000 people entered, as the Balanced Migration group advocate. That policy of “one in, one out” will not solve the problems of segregation, inequality, alienation and unhappiness that results from too much diversity (I’m not against cultural diversity, just too much of it, like I’m against too much of anything). Indeed, the most likely flashpoints of the future are among the least crowded areas in England, the former industrial towns of Yorkshire, Lancashire and the Midlands. It is demography rather than density that threatens social stability.

Certainly the Balanced Migration group are right to point to the dangers of over-population, and runaway immigration, but it is not the full story, and we should be honest about it. This morning the pressure group Sense About Science were on Radio 4 arguing – rightly, in my opinion – that in Britain scientific study and debate is hampered by our strict libel laws, and that we should be free to search for the truth, whether we find it comfortable or not. The same could be said about sociology and demography, except it is an unwritten code of politeness, embarrassment and social stigma, rather than libel law, that stops us being honest about how we run our small island.