The world's population should be at most 50 million.

[Mister Agenda]

That's the best part, if you don't want kids, you don't have to have them.  Doesn't take an incentive, doesn't require a poor fucking ideological commitment.  

I know..I know, you wont get all that sweet sweet free money the gov just gives us parenting types.  Imma gold plate my toilet this weekend with all that filthy lucre.

The worst part is that we know not enough people will choose to limit their reproduction soon enough to avert the dire consequences, which will fall disproportionately on the poor.

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See anybody forcing this?

Voluntary, capice??

Sure, sure....we can already get voluntarily sterilized, though..and theres no clear reason why the state would have a compelling interest in incentivizing that.  

Averting food and/or water shortages might be considered compelling by some state actors.

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Although personally, I think free birth control and sterilization would go a long way towards reducing family size of people in poverty, even without further incentive. I'd like to see numbers on that where it's been tried, though. Religion is likely to be a strong barrier to taking advantage of such programs.