Suppose you've got the government of a highly overpopulated nation like India, China or Somalia, and they want to reduce that population over time. How effective would it be to simply offer people a lump sum of money, or perhaps some stipend payable over time, like a mini-pension, if they submit to have a medical procedure performed on them that will prevent them ever being able to reproduce (but still enables them to have enjoyable sex, of course). It's an optional and free procedure for everyone, but you only get the stipend if you agree to do it before your first child, so you will never in your whole life have any children.
How many people do you think would take the offer? How attractive would the stipend have to be?