100% capitalism is the main source. And definitely not kill but engrave in people's minds that trying to have five children is selfish. If your action is the kind of action "if everyone does it, it is a big problem but because they don't I can do it" then there is a very good chance that it is a selfish behaviour. Reducing population will I think transform capitalist economy too. Less work force means higher demand for it and therefore companies will eventually have to pay higher wages. Less population means less demand for products and therefore capitalistic way of "cutting corners" mode of mass production will likely be not as profitable. Sure therewill always be people trying to capitalize on new means of production and yeah I also agree that changing the economic system will be required to prevent. But even changing the economic system will be easier with less population.