This study offers a broad view of the academic literature that deals with the effects of immigration on the sustainability of defined benefit PAYG social security systems. It is usually argued that a selective increase in the population helps towards the financial equilibrium of the system in the medium term.

However, in the long term, as immigrants adopt the demographic patterns of the native population, the situation reverts to the initial status. In other words, the entry of immigrants of working age improves the financial sustainability of the social security system in the short and medium term, since it postpones the explosion of the dependency rate; but those immigrants are also generating the right to future pensions, therefore transferring the financial problem to the next generation.