Talking about the "glass floor" which limits social mobility, Alan Milburn says:

It's a social scandal that all too often demography is still destiny.

This is true in a far bigger way than Mr Milburn means. There's a policy which suppresses social mobility and ensures that demography is destiny for millions of people.

I refer, of course, to immigration controls.

If you support social mobility you must, logically, favour open borders. For example, someone migrating from Zimbabwe, where GDP per head is barely $2000 per year, to a minimum wage job in the UK would see their income rise tenfold. That's massive upward mobility. And yet far from wanting to encourage such mobility, most people in the UK want to suppress it: a letter in the Telegraph today calls for water cannon to be used against migrants.

However, a world without immigration is one in which demography is destiny. People born in the UK will be at least ten times as rich as those born in Zimbabwe, regardless of merit or ability. This is a form of feudalism. Just as feudalists thought that people's fate should be determined by the station they were born into, so anti-immigrationists think our fate must be tied to the country we were born into.

This seems odd. If it's a "scandal" that demography is destiny then immigration controls must be scandalous. So why don't people believe they are?

I don't think the answer lies in conscious racism - that we want upward mobility for Brits but not Zimbabweans: I doubt that three-quarters of the British people are that racist.

One possibility is that one might favour open borders in principle but believe that they are simply impractical - especially for a single small country to adopt on its own. This, I think, is a reasonable position. But I'm not sure how widespread it is. I've not heard a politician say "I support open borders in principle; the problems are merely practical."

Another possibility is that some advocates of social mobility desire it not as a way of improving individuals' life-chances but as a way of justifying inequality; if everyone has a chance of success, then, it is thought*, inequalities are legitimate - as long as you don't think about poorer countries.

I suspect, though, that in most cases something else is going on. People just don't think what social mobility really means. Just as they don't appreciate that equality of opportunity requires a massive limitation of freedom - for example, banning private tuition - so they just don't see that social mobility must mean free migration. They are like those 18th century people who thought they were devout Christians but who kept slaves. It's easy to sustain two contradictory positions simply by not thinking.

* Very dubiously - but leave that aside.