The worldwide problem with pay-as-you-go (PAYG) social security systems isn't just financial. This study indicates that these systems may have exerted adverse effects on key demographic factors, private savings, and long-term growth rates. Through a comprehensive endogenous-growth model where human capital is the engine of growth, family choices affect human capital formation, and family formation itself is a choice variable, we show that social security taxes and benefits can create adverse incentive effects on family formation and subsequent household choices, and that these effects cannot be fully neutralized by counteracting intergenerational transfers within families. We implement the model using calibrated simulations as well as panel data from 57 countries over 32 years (1960-92). We find that PAYG tax measures account for a sizeable part of the downward trends in family formation and fertility worldwide, and for a slowdown in the rates of savings and economic growth, especially in OECD countries.