Melanie McDonagh: Soon there will be brothers and sisters again in China

China – at least, its biggest commercial city – is to modify its one-child policy. The authorities in Shanghai have declared that "eligible couples" – viz, two only-children – will be allowed, indeed, encouraged, to have a second child.

Just don't run away with the idea that this is an outbreak of liberal, pro-choice individualism on the part of the authorities. As Xie Linli, the director of the Shanghai Population and Family Planning Commission, said: "We advocate eligible couples to have two kids because it can help to reduce the proportion of the ageing people and alleviate a workforce shortage in the future."

Mmm. So the obvious downside of the one-child policy has only just occurred to them? Population growth is a bit like a pyramid selling scheme: unless you've got income coming in at the bottom, you can't continue paying out to those at the top.

Regardless of its obvious economic advantages – and the proportion of over-60s in Shanghai, now more than a fifth, will rise to a third by 2020 – it's hard to think of a measure that will do more, at a stroke, to increase the sum of human happiness, apart from letting people decide the size of their families for themselves. And that's something the Chinese have not been allowed to do for decades: the opposite of the one-child policy was Mao's previous insistence that the Chinese should have large families. The upshot is, there'll be more people who know what the words sister and brother mean in a human, not linguistic, sense. There'll be Chinese in a generation's time who will know what it is to have an aunt or uncle; cousins, even.

This is not to say that I don't see the advantages of only children. I and half the population of China have that much in common. Being an only child means that your place in the universe is secure. You don't have any of the tiresome competition for attention. There's no squabbling about the pecking order, or inheritance. You are, in your own family at least, the Little Emperor.

Spoilt, I grant you, and occasionally lonely, but with the internal self-sufficiency that comes from having to amuse yourself quite a lot of the time. As Wilkie Collins put it, in his novel, No Name: "Miss Marrable was that hardest of all born tyrants – an only child. She had never granted a constitutional privilege to her oppressed father and mother, since the time when she cut her first tooth."

From the parents' point of view, the advantages are equivocal, apart from the purely selfish circumstance that you have more of your own money. Your hold on posterity is fragile. One of the unbearable implications of last year's Chinese earthquake was that so many of the casualties – perhaps 68,000 – would have been only children. A society where the old exceed the young is unbalanced.

In Confucian terms, the individual's pre-eminent obligations are to parents, grandparents and ancestors. Now we have something like descendant worship.

The partial reversal of the one-child policy – which was enforced by compulsory abortions and involuntary sterilisations – will have an environmental impact on the rest of us. But we should give those environmentalists who rejoice in draconian population policies for other people very short shrift.

I've come across any number of supposedly liberal academics who, sotto voce, will tell you of the immense benefits of China's one-child policy. Well, they'll just have to come to terms with this advance in freedom. For anyone who cares at all about human liberty, it's something we should celebrate.