In the last century, state pension systems have been introduced in most countries, and since then their size has been significantly increasing. A broad literature has studied this phenomenon, developing models that explain why pension systems exist and have been continuously expanding. At the same time, many authors have suggested that pension systems may substitute children as old-age economic security, discouraging fertility. In particular, this fact may explain the contemporaneity of the expansion of pension systems with the urbanization and industrialization processes. These two processes, in fact, have contributed to the weakening of family ties, which in turn results in the need for additional old-age economic security. In the political economy research these effects have been ignored, as the fertility choice is usually considered exogenous. This paper suggests a model that takes into account this endogenous effect and tries to analyze the net effect of the breakdown of family ties on the dimension of pension systems. The last section presents some empirical results supporting the theoretical model. The main result is that the transition toward the weak family does not necessarily imply an increase in the size of pension systems, because as the family structure becomes weaker the fertility decreases, thus reducing the profitability of the scheme: as the weak families start to increase, there could be an increase in the size of the pension system, but as they become the majority, the fertility rate may become too low, and the political support for the pension system may decrease.