A lot of folks in the circles that I blog in appear to believe that a return to Malthusian constraints is a feature solely of dark futures or futures within which technology stagnates, perhaps due to increasingly complex and oppressive regulatory schemes strangling individual initiative. I don't rule such futures out, or even estimate that their probability is low. In fact I view such futures as collectively possessing a better than even probability of coming to pass. But, as I'm quite optimistic by nature, I think that a bright future, something akin to what many futurists would call a positive singularity, has a decent chance to happen as well---I need not even a few beers to take the possibility seriously, much less more potent mind-altering chemicals.
What do I mean by a bright future? Well, here's a start:
Actuarial escape velocity: essentially the state wherein your remaining time to live is increasing at a rate faster than time is passing.
Presently lifespans in the developed world are increasing fast enough to give actuaries fits in computing correct annuity payouts for the very aged. Assuming we don't kill the goose that lays the golden eggs, I see this having a decent probability of happening sufficiently soon to benefit many of my readers, and certainly our children.
To be a bright future, we'd also have to solve our energy availability per capita problem (available energy per capita peaked if I recall in the early 1970s), which requires some rabbit from a hat produced by our engineers or scientists. It'll need to be a damned big rabbit, quite probably some flavor of fusion coupled most likely with significant advances in superconductivity and/or battery technology energy density. Presently, since around 2005, we've been on a plateau of oil (and other liquid fuels) production and we may well have in fact hit a peak. Cheap energy is in fact what makes the world as we know it go around and this one is really the hardest one technologically to get by on the way to a positive singularity.
We'd also have to insure that the center does in fact hold long enough for us to solve the problems cited above. This means, for instance, that the US gets itself onto a sustainable fiscal footing. One can only extend and pretend so long, even when one holds the world's reserve currency. This part of the problem isn't hard technologically, but it is IMO the hardest part of the problem.
All told, my gut estimate is that the chances of something like the above coming to pass is in the neighborhood of 10-25%...probable enough that I at least consider it in my future planning. But I'm something of an optimist, as I've mentioned before.
Let's say that in future generations we manage to convert the solar system and areas immediately around it into something like a Niven-esque Ringworld.
That's an awful lot of carrying capacity for population---approximately 3 Million Earths worth. If we assume a comfortable 33 Billion per Earth equivalent, somewhere around 4-5x our present density, we're talking a capacity for 100*10^15 human beings. That's a hell of a lot---100 quadrillion. How does one have a rendezvous with Malthus in such a science fiction scenario?
Well, we've been running an experiment for about one hundred years now. Our experiment is to identify and purify strains of humanity that are resistant to the fertility depressing aspects of modernity. I'm on personal terms with some of the first fruits of this new strain of humanity. I fully expect that he'll have close to 100 grandchildren before he passes from the Earth and God knows how many great grandchildren. Consider this---10 billion human beings who collectively have a rate of increase of population that just barely balances losses through accidents, suicide, foul play, and the like occupy this Ringworld. In addition, a small number of couples who have just four children, and whose children inherit their particular level of fertility over time, with the aid of nature and homeschooling, with an average generation time of 20 years. For simplicity, we'll assume that despite having achieved Methuselah levels of longevity, only one batch of four children is produced by each couple.
Each generation, this group doubles its numbers and roughly its proportion of the total population. In 100 years, this amounts to 5 doublings. Every 200 years, this is 10 doublings, which is close enough to 1000 for government work. In 1000 years, this is 1000^5 which is...guess what...1 quadrillion. By this point it should be pretty obvious that this group will gain demographic hegemony pretty quickly (relative to the time horizons likely to persist in a world where actuarial escape velocity has been achieved) if unchecked. This is the reason why I suspect that in such a future, external coercive controls on fertility are likely to be imposed. This is also why I encourage my allies, witting or unwitting, to go ahead and have more children now if they desire them. Such controls are exceedingly unlikely to take into account what is already 'baked into the cake' so to speak. This position is the same position that I have for collapsed or stagnated possible futures, albeit for different reasons