|Also, I've read that people like maths-themed pictures of kittens.|
Let's start with a simple model: We have two sets of babies, R rich babies and P poor babies. Rich babies each consume S resources, while poor babies each consume Q resources. There are T total resources in the world. Obviously, T must be greater or equal to (P.Q) + (R.S), and so the upper limits of P, Q, R and S are all restricted by each other. Because Q is defined as a small number, P can be larger. S is defined as a large number, so R must be smaller. But P and R also limit each other, as do Q and S. Because, no matter what, you can't ever have more stuff than T.
So there's competition, even when we make things more realistic and consider all the ways in which PQRST can be more subtle and complicated. That basic model can be manipulated and expanded by thousands of extra variables, but the basic point it illustrates remains true: You can't have more stuff than exists, so more people using more stuff means more pressure on the supply of stuff. And when (P.Q) + (R.S) ends up greater than T, the only possible result is starvation and death.
My whole life, I've heard from (mostly well-meaning) people that poor people disadvantage themselves by having more children than they can afford to support, perpetuating a cycle of poverty, because those under-resourced children are ill-equipped to compete with the health, education and general supporting mechanisms the rich kids have.
This is, thinking it over, a very sad version of victim-blaming. If S = 10.Q, then one rich baby is effectively depriving 9 poor babies of their resources. If S = 1000.Q, then our rich baby has stolen from 999 poor babies! If parents with only Q resources have 2.P babies, they have deprived half their babies (or more realistically, partially deprived all their babies), but that's still a 1 to 9 ratio of over-consumption compared with the S = 10.Q rich babies. Put that way, it's entirely possible that rich parents are the ones we should be condemning for having so many over-indulged babies that we collectively can't afford. The cycle of poverty can be perpetuated by people who aren't poor and may never be poor.
Filling in real-world numbers is a little trickier, because we need to define terms and shit, and you could argue over exactly how to make things fit best. But as a quick, simple demonstration, I'm just going to yank similar-sized national populations with very different GDP per capita (from here, noting that it's 2008 data) and make the crude assumptions that GDP per capita is uniform within each state, and represents solely and entirely the resources available to all their current babies. There are definitely more appropriate and specific figures, instead of those, but I'm only trying to show that a rough approximation is possible that can be assembled by non-specialists with limited familiarity and limited research, which still shows the same basic pattern of results.
I pick Tanzania (P = 35 million humans, Q = $500) and South Africa (R = 46 million humans, S = $29 000), and the GDP per capita alone immediately shows that Q and S can be vastly different in reality. For those without calculators at home, this S = 58.Q, indicating that one South African kid is taking up resources that 57 additional Tanzanian kids would be using, on average. If we take the UN's dollar-a-day minimum survivable income guideline, we can see that Q just barely beats this bedrock minimum of $365. Since P is sort of close to Q, we can further say that even if the Tanzanian population doubled overnight, that population pressure would still be less harmful to Tanzanian babies than the resource competition pressure from South Africa. In this simple case, Tanzania is taking (P.Q = 17,5 billion) units from T, while South Africa takes up (R.S = 1 334,0 billion) units, 76 times more. If T were exactly 1 trillion units, the shared shortfall would hurt South Africa, but it would wipe out Tanzania utterly. Defining the actual value of T, deciding what we should and shouldn't include in it, is another frustration I will gloss over for today; just accept that it's not a very flexible figure at the best of times.
(Of course, reality is even more granular and messy, with stark inequality within both those countries. We could draw an even more dramatic comparison, two decades after apartheid was officially voted out, between the top average white male income and the bottom average black female income in South Africa alone. And then there's the whole rest of Earth to look at. We could discuss the finer details for decades and still not cover it all. Hence my focus on the big picture.)
Obviously I'm looking at this through somewhat Marxy-tinted glasses, but even if you want to take a strictly capitalist-meritocratic view, that different work merits different pay, then please remember that those R babies have done sweet fuck all to earn their wealth, and the P babies haven't yet had a chance to test their merit at all either. I don't believe there is any coherent argument for intentionally keeping the value of S higher than Q, other than naked (and deeply instinctive) self-interest. And that's not a good thing.
There are, of course, still good reasons for cutting over all population growth among everyone, rich and poor. The smaller the sum of P and R, the bigger the sum of Q and S can be, plus we benefit from enormous and fantastic ecological and technical benefits if we totally arrest the growth of P+R. And individuals should certainly do the basic personal maths on their own household economics, as one part of the decision to make any new human. But that personal capacity can't be the only thing we consider. A wider societal perspective is also important - more important, I argue!
So what do we do with this model?
Any sane, self-interested potential parent would want their kid(s) in the R population, and they'd want S to be as high as possible. And most people will openly agree to that, I'm sure. What fewer people will be open about, however, is whether they accept that P and Q will have to shrink to accommodate their private desires. In my experience, mostly talking to middle and upper income people, their public discussion of this comes down to ideology, but regardless of that, very few seem to be willing to admit that R fucking things up for P is a problem that they personally will change their behaviour to fix.
But I think the maths is clear. If we're not assholes, we want Q = S, so that all kids have as close to an equal and fair start in life. And until Q does really equal S, then every growth in R just makes the situation worse, at a worse rate than any growth in P.
I have a lot of wealthy friends and acquaintances* in the the age bracket where humans usually settle down and breed, and these people are basically good people. These people are mostly well educated; some are extremely well educated and unusually intelligent. (Most are atheist, incidentally, so religion is only a distant factor in this discussion.) Almost invariably, they've almost all been very eager to agree with me that overpopulation is a problem and that fewer people should breed and should be having fewer babies when they do (with the exception of one professional philosopher, who started from an argument from ignorance to claim that he didn't know - and wasn't willing to make any effort to learn - any reason for him not to breed, which was disappointing). Usually, with most of these people, we're all on the same page. And then they have a kid, explaining it as "well, we're only having the one," apparently totally oblivious to any inkling of the maths above. And then they have a second one, with the excuse "we're only having two, one to replace each parent," as if we actually want or need the parents replaced. (The other common yet moronic excuse for the second kid is that the first "needs a friend", as if the 7 billion potential friends who already exist count for nothing.) And then they spit out a third kid and all I can think is "Oh, fuck the fuck off, you selfish fucking fuckers!" But society tells me its rude to swear at people when they announce a pregnancy, so I usually restrain myself.
My point here is that there's a lot of irrational, instinct-driven hypocrissy around pregnancy and breeding, and we need to change what is socially acceptable to fix this. And I don't mean the swearing rules, I mean what is socially acceptable in terms of who should demand the right to breed and who we should be condemning when they choose to.
Whether we have formal pregnancy limit laws or vague social norms and mores on breeding, I think my bottom line is that it's important that we consider the global impact of the child's consumption on the world, and not only the local impact of the child's consumption on the child. From that point of view, the rich(er) giving the poor(er) shit because they had kids is clearly inappropriate, and instead the richest should be most worrying to us.
Incidentally, we can easily extend this line of thought to also cover non-human breeding. The environmental impact of the meat industry is massive, and there simply is no way to "fix" that with the current human population size. Even if you don't understand the individual-level animal rights arguments, it's just simple maths again to understand that X humans need Y plants to survive, while X humans need Z animals to survive, all of which will each need Y plants to stay alive until they get killed. And X.Y will always be smaller than X.Y.Z. But again, people** would rather insist that we breed more cows, more pigs, more sheep, more chickens, more gagh, anything at all, regardless of the costs, rather than admit that we can get by fine without eating meat. And this has the further effect of pushing up the value of S for no good reason, which adds to how much the P population gets fucked.
*Increasingly, it's become fewer friends and more acquaintances. People take this stuff incredibly, deeply personally. I've had conversations with otherwise reasonable, easy-going people who take my suggestion that a hypothetical person should hypothetically choose not to have any children, and somehow twist that through parental animal instinct into believing I've said we should take their actual, current, already-born children away from them, to roast them and eat them. And I only slightly exaggerate that interpretation. Obviously, those humans who already exist should be treated as well and fairly as possible (speaking as a human who already exists), but that's a totally separate question from whether we should, in principle, tolerate careless breeding and population growth before it's happened, in the planning stages. I guess people also don't like admitting they've made irreversable mistakes, especially on that scale.
**People take meat-eating almost as personally as breeding. It's weird.