A third, more anarchic way to transfer resources from young to old is a speculative bubble.
In a bubble, people pay over the odds for an asset, such as a house, in the belief that subsequent investors will pay a higher price still.
The overpayment amounts to a contribution to a Ponzi scheme, redeemed not by the earnings of the underlying asset, but by overpayments from later investors.
If each generation is collectively richer than the last, then the asset's price can keep rising even if each buyer sinks only the same percentage of their (rising) income into it.
All these mechanisms have side-effects.
Government debt can crowd out productive investment.
Bubbles can do the same.
But in an economy suffering from dynamic inefficiency, this crowding out is a good thing.
Such an economy has accumulated too much capital.
It requires heavy investment to keep this large stock of machinery, equipment, buildings and infrastructure growing in line with the economy.
The young are tempted to add yet more capital in an effort to transfer resources to their future, older selves.
Yet because the returns are so low, it is more efficient for them to transfer resources directly to today's elderly (by contributing to social security, buying government bonds or overpaying for the old people's assets), on the understanding that tomorrow's young will do the same for them.
Under certain conditions, then, Ponzi principles are efficient not maleficent.
But are those conditions ever found in the real world?
One way to look for them is to compare interest rates (which represent the return on capital) and GDP growth rates (which reflect both population growth and increases in income per person).
An alternative, stiffer test (which works well under certain assumptions) is to compare investment and profits.
If national investment is greater than profits, a country is ploughing more into its stock of capital than it earns from it.
It is as if the islanders are replanting all of the fruit they collect from the additional trees they have cultivated (minus whatever fruit they need to compensate themselves for their labour) plus a few more.
The economy's efforts to save and invest for the future are overloading the available vehicles for thrift.
Economists used to be confident that most economies were on the right side of this test, earning far more in profit than they invested.