In other words, when it becomes clear that the next generation will be worse off than the current generation because there are too many people, it is very likely that anti-fertility memes and possibly laws will become prevalent.
I don't know about laws, but I don't see why “high fertility decreases happiness of humans” necessarily implies “low-fertility–causing memes have a reproductive advantage”. Nor why that advantage would shadow that of other memes.
Insofar as people are mortal, significant changes in fertility and population can happen extremely quickly.
Local (time-wise) changes can be quick and large in any direction, true, but I thought the argument was that as long as fitness selection works, in the long term, it always finally pushes them back towards the Malthusian limit.
So even in an extremely sane and hedonistic population (say, immortals who mostly never choose to reproduce), as long as some inheritable trait (genes, memes or something else) [a] increases the likeliness of reproduction relative to the average and [b] varies among the population, eventually individuals with that trait will dominate because... well, they're the only ones who reproduce.
(Of course, if society can perfectly control all such factors, then the problem disappears. It's just that it doesn't seem possible even in theory.)
This is an attempt to list all of the possible ways in which humanity may avoid scenarios where the average standard of living is close to subsistence, in response to Robin Hanson's recent series of posts on Overcoming Bias, where he argues that such an outcome is likely in the long run.
I'll start with six, some suggested by myself, and others collected from comments on Overcoming Bias and Robin's own posts. If anyone provides additional ideas, I'll add them to the list.
(I have a more general point here, BTW, which is that predicting the far future is very difficult. Before thinking that some outcome is inevitable or highly likely, it's a good idea to repeatedly ask oneself "This is all the ways that I can think of why it may fail to come true. Am I sure that all of them have low probability and that I'm not missing anything?" There may be some scenario with a non-negligible probability that your brain simply overlooked when you first asked it.)
Singleton
A world government or superpower imposes a population control policy over the whole world.
Strong Security
Strong defensive technologies and doctrines (such as Mutually Assured Destruction) allow nations, communities, and maybe tribes and families to unilaterally limit their populations within their own borders, while holding off hordes of would-be invaders and immigrants.
Non-Human Capital
Maximizing the wealth and power of a nation requires an optimal mix of human and non-human capital. Nations that fail to adopt population controls find their relative wealth and power fade over time as their mixes deviate from the optimum (i.e., they find themselves spending too much resources on raising humans, and not enough on building machines), and either move to correct this or are taken over by stronger powers. (I believe that historically this was the reason China adopted its one-child policy.)
Unlimited Growth
We don't completely understand the laws of physics, nor the nature of value. There turns out to be some way for economic growth to continue without limit. (Robin himself once wrote "I know of no law limiting economic value per atom" but apparently changed his mind later.)
Selfish Memes
Memes that manage to divert people's resources away from biological reproduction and towards memetic reproduction will have an advantage over memes that don't. On the other hand, genes that manage to block such memes will have an advantage over genes that don't. Memes manage to keep the upper hand in this struggle (or periodically regain the upper hand).
Disease, Warfare, Natural Disasters, Aliens, Keeper of the Simulation
One or more of these come along regularly to keep the human population in check and per capita incomes above subsistence.