The outline of the changes needed is clear. To avoid fiscal （财政的） meltdown， public pensions and health-care provision will have to be reined back severely and taxes may have to go up. By far the most effective method to restrain pension spending is to give people the opportunity to work longer, because it increases tax revenues and reduces spending on pensions at the same time. It may even keep them alive longer. John Rot her, the AARP's head of policy and strategy, points to studies showing that other things being equal, people who remain at work have lower death rates than their retired peers.
Younger people today mostly accept that they will have to work for longer and that their pensions will be less generous. Employers still need to be persuaded that older workers are worth holding on to. That may be because they have had plenty of younger ones to choose from, partly thanks' to the post-war baby-boom and partly because over the past few decades many more women have entered the lab our force, increasing employers' choice. But the reservoir of women able and willing to take up paid work is running low and the baby-boomers are going gray.
In many countries immigrants have been filling such gaps in the lab our force as have already emerged （and remember that the real shortage is still around 10 years off）. Immigration in the developed world is the highest it has ever been, and it is making a useful difference. In still fertile America it currently accounts for about 40% of total population growth, and in fast-ageing we stern Europe for about 90%.
On the face of it, it seems the perfect solution. Many developing countries have lots of young people in need of jobs; many rich countries need helping hands that will boost tax revenues and keep up economic growth. But over the next few decades lab our forces in rich countries are set to shrink so much that inflows of immigrants would have to increase enormously to compensate: to at least twice their current size in western Europe's most youthful countries, and three times in the older ones. Japan would need a large multiple of the few immigrants it has at present. Public opinion polls show that people in most rich countries already think that immigration is too high. Further big increases would be politically unfeasible.
To tackle the problem of ageing populations at its root, "old" countries would have to rejuvenate （使年轻） themselves by having more of their own children. A number of them have tried, some more successfully than others. But it is not a simple matter of offering financial incentives or providing more child care. Modern urban life in rich countries is not well adapted to large families. Women find it hard to combine family and career. They often compromise by having just one child.
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