https://en.wikipedia.org/wiki/PAYGO#Social_insurance

--------------------------------------------------
Note added at 21 mins (2016-02-08 15:59:51 GMT)
--------------------------------------------------

https://fr.wikipedia.org/wiki/Retraite_par_répartition

--------------------------------------------------
Note added at 28 mins (2016-02-08 16:07:00 GMT)
--------------------------------------------------

Defined benefit plans may be either funded or unfunded.

In an unfunded defined benefit pension, no assets are set aside and the benefits are paid for by the employer or other pension sponsor as and when they are paid. Pension arrangements provided by the state in most countries in the world are unfunded, with benefits paid directly from current workers' contributions and taxes. This method of financing is known as Pay-as-you-go (PAYGO or PAYG).[13] The social security systems of many European countries are unfunded,[14] having benefits paid directly out of current taxes and social security contributions, although several countries have hybrid systems which are partially funded. Spain set up the Social Security Reserve Fund and France set up the Pensions Reserve Fund; in Canada the wage-based retirement plan (CPP) is partially funded, with assets managed by the CPP Investment Board while the U.S. Social Security system is partially funded by investment in special U.S. Treasury Bonds.

In a funded plan, contributions from the employer, and sometimes also from plan members, are invested in a fund towards meeting the benefits. All plans must be funded in some way, even if they are pay-as-you-go, so this type of plan is more accurately known as pre-funded. The future returns on the investments, and the future benefits to be paid, are not known in advance, so there is no guarantee that a given level of contributions will be enough to meet the benefits. Typically, the contributions to be paid are regularly reviewed in a valuation of the plan's assets and liabilities, carried out by an actuary to ensure that the pension fund will meet future payment obligations. This means that in a defined benefit pension, investment risk and investment rewards are typically assumed by the sponsor/employer and not by the individual. If a plan is not well-funded, the plan sponsor may not have the financial resources to continue funding the plan. In many countries, such as the USA, the UK and Australia, most private defined benefit plans are funded[citation needed], because governments there provide tax incentives to funded plans (in Australia they are mandatory). In the United States, non-church-based private employers must pay an insurance-type premium to the Pension Benefit Guaranty Corporation (PBGC), a government agency whose role is to encourage the continuation and maintenance of voluntary private pension plans and provide timely and uninterrupted payment of pension benefits. When the PBGC steps in and takes over a pension plan, it provides payment for pension benefits up to certain maximum amounts, which are indexed for inflation.[1]