Max Tegmark observed that we have three independent reasons to believe we live in a Big World:  A universe which is large relative to the space of possibilities.  For example, on current physics, the universe appears to be spatially infinite (though I'm not clear on how strongly this is implied by the standard model).

If the universe is spatially infinite, then, on average, we should expect that no more than 10^10^29 meters away is an exact duplicate of you.  If you're looking for an exact duplicate of a Hubble volume - an object the size of our observable universe - then you should still on average only need to look 10^10^115 lightyears.  (These are numbers based on a highly conservative counting of "physically possible" states, e.g. packing the whole Hubble volume with potential protons at maximum density given by the Pauli Exclusion principle, and then allowing each proton to be present or absent.)

The most popular cosmological theories also call for an "inflationary" scenario in which many different universes would be eternally budding off, our own universe being only one bud.  And finally there are the alternative decoherent branches of the grand quantum distribution, aka "many worlds", whose presence is unambiguously implied by the simplest mathematics that fits our quantum experiments.

Ever since I realized that physics seems to tell us straight out that we live in a Big World, I've become much less focused on creating lots of people, and much more focused on ensuring the welfare of people who are already alive.

If your decision to not create a person means that person will never exist at all, then you might, indeed, be moved to create them, for their sakes.  But if you're just deciding whether or not to create a new person here, in your own Hubble volume and Everett branch, then it may make sense to have relatively lower populations within each causal volume, living higher qualities of life.  It's not like anyone will actually fail to be born on account of that decision - they'll just be born predominantly into regions with higher standards of living.

Am I sure that this statement, that I have just emitted, actually makes sense?

Not really.  It dabbles in the dark arts of anthropics, and the Dark Arts don't get much murkier than that.  Or to say it without the chaotic inversion:  I am stupid with respect to anthropics.

But to apply the test of simplifiability - it seems in some raw intuitive sense, that if the universe is large enough for everyone to exist somewhere, then we should mainly be worried about giving babies nice futures rather than trying to "ensure they get born".

Imagine taking a survey of the whole universe.  Every plausible baby gets a little checkmark in the "exists" box - everyone is born somewhere.  In fact, the total population count for each baby is something-or-other, some large number that may or may not be "infinite" -

(I should mention at this point that I am an infinite set atheist, and my main hope for being able to maintain this in the face of a spatially infinite universe is to suggest that identical Hubble volumes add in the same way as any other identical configuration of particles.  So in this case the universe would be exponentially large, the size of the branched decoherent distribution, but the spatial infinity would just fold into that very large but finite object.  And I could still be an infinite set atheist.  I am not a physicist so my fond hope may be ruled out for some reason of which I am not aware.)

- so the first question, anthropically speaking, is whether multiple realizations of the exact same physical process count as more than one person.  Let's say you've got an upload running on a computer.  If you look inside the computer and realize that it contains triply redundant processors running in exact synchrony, is that three people or one person?  How about if the processor is a flat sheet - if that sheet is twice as thick, is there twice as much person inside it?  If we split the sheet and put it back together again without desynchronizing it, have we created a person and killed them?

I suppose the answer could be yes; I have confessed myself stupid about anthropics.

Still:  I, as I sit here, am frantically branching into exponentially vast numbers of quantum worlds.  I've come to terms with that.  It all adds up to normality, after all.

But I don't see myself as having a little utility counter that frantically increases at an exponential rate, just from my sitting here and splitting.  The thought of splitting at a faster rate does not much appeal to me, even if such a thing could be arranged.

What I do want for myself, is for the largest possible proportion of my future selves to lead eudaimonic existences, that is, to be happy.  This is the "probability" of a good outcome in my expected utility maximization.  I'm not concerned with having more of me - really, there are plenty of me already - but I do want most of me to be having fun.

I'm not sure whether or not there exists an imperative for moral civilizations to try to create lots of happy people so as to ensure that most babies born will be happy.  But suppose that you started off with 1 baby existing in unhappy regions for every 999 babies existing in happy regions.  Would it make sense for the happy regions to create ten times as many babies leading one-tenth the quality of life, so that the universe was "99.99% sorta happy and 0.01% unhappy" instead of "99.9% really happy and 0.1% unhappy"?  On the face of it, I'd have to answer "No."  (Though it depends on how unhappy the unhappy regions are; and if we start off with the universe mostly unhappy, well, that's a pretty unpleasant possibility...)

But on the whole, it looks to me like if we decide to implement a policy of routinely killing off citizens to replace them with happier babies, or if we lower standards of living to create more people, then we aren't giving the "gift of existence" to babies who wouldn't otherwise have it.  We're just setting up the universe to contain the same babies, born predominantly into regions where they lead short lifespans not containing much happiness.

Once someone has been born into your Hubble volume and your Everett branch, you can't undo that; it becomes the responsibility of your region of existence to give them a happy future.  You can't hand them back by killing them.  That just makes their average lifespan shorter.

It seems to me that in a Big World, the people who already exist in your region have a much stronger claim on your charity than babies who have not yet been born into your region in particular.

And that's why, when there is research to be done, I do it not just for all the future babies who will be born - but, yes, for the people who already exist in our local region, who are already our responsibility.

For the good of all of us, except the ones who are dead.