In the roughly 250 years since the Industrial Revolution the world's population，like its wealth, has exploded. Before the end of this century，however，the number of people on the planet couldshrink(缩小) for the first time since the Black Death. The root cause is not an increase in deaths, but a sharp fall in births. Across much of the world the fertility rate， the average number of births per woman，is collapsing. Although the trend may be familiar，its extent and its consequences are not. Even as artificial intelligence( AI) leads to optimism in some quarters， the baby bust(婴儿荒) hangs over the future of the world economy.
The result is that in much of the world，the patter of tiny feet is being drowned out by the clatter of walking sticks. The prime examples of ageing countries are no longer just Japan and Italy but also include Brazil，Mexico and Thailand. By 2030 more than half the inhabitants of East and South-EastAsia will be over 40. As the old die and are not fully replaced，populations are likely to shrink. Outside Africa，the world's population is forecast to peak in the 2050s and end the century smaller than it is today. Even in Africa，the birth rate is falling fast.
Whatever some environmentalists say，a shrinking population creates problems. The world is not close to full and the economic difficulties resulting from fewer young people are many. The obvious one is that it is getting harder to support the world's pensioners(养老金). While the rich world currently has around three people between 20 and 64 years old for everyone over 65, by 2050 it will have less than two. The implications are higher taxes，later retirements，lower real returns for savers and, possibly，government budget crises.
Eventually，therefore, the world will have to make do with fewer youngsters—and perhaps with a shrinking population. With that in mind，recent advances in AI could not have come at a better time. Al economy might find it easy to support a greater number of retired people. Eventually AI may be able to generate ideas by itself，reducing the need for human intelligence. Such innovations will certainly be in high demand.
