  Birth rates have dramatically decreased and, with continuous improvements in
life expectancy, pension expenditure is on an irreversibly increasing path.
This will raise serious concerns for the sustainability of the public pension
systems usually financed on a pay-as-you-go (PAYG) basis where current
contributions cover current pension expenditure. With this in mind, the aim of
this paper is to propose a mixed pension system that consists of a combination
of a classical PAYG scheme and an increase of the contribution rate invested in
a funding scheme. The investment of the funding part is designed so that the
PAYG pension system is financially sustainable at a particular level of
probability and at the same time provide some gains to individuals. In this
sense, we make the individuals be an active part to face the demographic risks
inherent in the PAYG and re-establish its financial sustainability.
