Orange Room Supporter
It works the same way as the pensions I spoke about. Government pension is a defined benefit plan. The kinks may differ, but the concept is the same. In the US (and anywhere else I reckon), pension plans receive most of their annual income from investments, not contributions. See figure.
Your assertion is not accurate. (The anywhere else assertion). In France, all benefits are funded by pay from existing taxpayers. In fact, even in the US social security is funded by existing tax payer (that 6% you pay on your income). The issue is that the benefits are so low that all those who can afford it and are salaried employees complement it with either a defined benefit plan (slowly evaporating) or a defined contribution plan.