the 2-3 they have will survive.

Also when more kids = more economic liability. If you're agrarian your offspring are your workforce, but only if the kids work instead of going to school. School kids are an economic liability to a family. It usually takes cultural perspectives a generation or two to change when economics changes; iron-clad, fixed-in-cement cultural traits can make that harder (so if every week some preacher says you need a lot of kids and that's the dominant or state religion...).

And child limitations are the biggest way to fight poverty in the 3rd world. If you are married with two kids you need a certain amount of money to live. For each kid, you need more income. When you see a family with 4, 5, 6, 7 kids that isn't primarily agrarian with stay-at-home kids, you're probably looking at a poor family.

There's a bit of a hazard here, because if you just remove the economic liability there's no reason for the culture to change. Lots of kids show that you're a man or a real mother. It's your obligation, your duty, and if there's not much of a downside that won't change.

This has happened in some parts of the world. There are a few areas where even if the crops do not fail because of drought or war the land cannot sustain the primarily rural and agrarian population. Periodic droughts and famines used to knock down the population; in the last 50 years the population's justed increased and during famines held steady. I'm not talking about cities. Imagine Iowa so crowded that it couldn't feed itself. Either the population has to move or that area will forever need imports of food.

to be a big issue, especially with global warming. A lot of places are going to be uninhabitable, at least for poor farmers, and they're going to need a place to live. A lot of cities are pricing average income and poor people out. The world is changing quickly, and we're going to have to adapt like we always have. It's going to be a difficult process for many of us.

Birth control should be encouraged always. We need to find ways to make limiting family size a more lucrative choice for people. Really, having a large family these days, with what we've learned through the years about the planet and man's effect on it's natural processes, is a bit selfish. Certain people think that they are so wonderful and special that they need to pass on their genes to the pool. They usually are the last people you'd want contributing to the gene pool. Maybe we should give a check to people who have one kid. We need to do something, because right now it's out of control

And subjective.

Also difficult to have a worldwide movement to end hunger, disease, and poverty on a finite planet. At least in the way that would satisfy our potential obligation. That's why our environmental issues are as complex as they are. Every plus comes with a minus.

and family.

If liberals don't do what we can, no one will help and we are effectively endorsing the "bootstraps" strategy for alleviating global poverty.

...the rest of the world?

Seems a pretty damn arbitrary definition of 'justice' in that case.

But really you're back to the old Colonialism question. Do we have a responsibility to use our culture - because it's the best, just ask us! - and civilization to enforce our current beliefs in human rights and economic justice across the whole world because it will save lives of those who are (undoubtedly) oppressed? I think we like to say "NO, OF COURSE NOT!"...until a question of humanitarianism comes up, then we like to forget the original answer until it's gone again. I think the real answer is somewhere in the "Yeah...sort of, and not really at the same time. It depends on a lot of other things." department, but that just tends to anger both sides.