Any suggestions on how to get the human population down to 500 million?
It's as simple as providing reliable birth control with increased education and economic opportunities to women in the third world.
Why would they care about increased economic opportunities if they can't have kids? I'm sure there's a simple solution to that, and I do rather like the idea of making the system voluntary, at least at first.
Of course the right-wing and various fundamentalists won't go for the policy, so the initial cohort leaving the gene pool will all be good progressives and the poor people who are desperate enough to fall for food in return for "reliable" birth control. Then, once those are too old to breed and finally die off, the rest of us will sit back on the beach and listen to Rush Limbaugh telling us what a true work of genius the policy was.