Genes, Memes, and the Population Problem
The main explanation for the decline seems to be a mixture of the economic, the technological, and the social.
The economic argument blames the fact that kids are a lot of cost and not a lot of monetary benefit, in stark contrast to agrarian societies, where kids were a useful source of cheap labor on the farm. This makes a certain level of sense, but then how to account for the fact that high birthrates persisted well into the industrial revolution? Granted that many lower-class families sent their kids to work in the factories, but this practice was abolished (at least in the West) around the turn of the last century, and birthrates stayed high until relatively recently.
The technological argument is basically the birth control argument: in the age of the Pill, people don't hafta have kids unless they wanna. Once again, this is part of the story, but it can't be everything: infanticide was an easily available - and widely used - practice throughout much of history, and even beyond that women since time immemorial have known of plenty of ways (herbal remedies, various excercises) that could be used to abort an unwanted child, or at least raise the probability of miscarriage. They could and did do this, in order to prevent children being born out of wedlock or into families too poor to support another hungry mouth (even on the farm, after all, kids aren't much good until they're three or four, and in lean years that short time can make a big difference.)
The social argument comes down to the emancipation of women. In almost every previous society, the primary role of women - due to unavoidable biological realities - was reproduction. Now that women have stormed the bastions of almost every male sanctum, they're too busy with careers to bother with kids. Despite the dirty looks I get in public when I say such things (at least from women) this is, I think, the hardest to refute. Still, though, one has to ask the question of why, exactly, women decided that now was the time to climb the corporate ladder? The usual explanation is that in previous societies the 'patriarchy' kept women down, but while there's plenty of evidence for patriarchy, there isn't so much evidence that previous generations of women were all that interested in doing guy stuff.
Now, it's obvious that none of these three theories are mutually exclusive, and may well be mutually reinforcing. Chemical birth control and safe abortions are undoubtedly new; modern economies certainly do inhibit child-rearing, to the extent that they make kids massively expensive. The former trend makes it easier for women to put off kids indefinately, while the latter discourages having kids at all. Add in a healthy dollop of ideological feminism (but once again, Why did that ideology thrive in the modern environment?) and you'd seem to have a complete explanation.
I used to be more or less satisfied with that smorgasbord, at least, until I came upon a slightly different explanation in Susan Blackmore's fascinating The Meme Machine. For anyone unacquainted with memetics (and I'll assume that's almost everyone, despite 'meme' having become one of the internet's favorite buzzwords), here's a crash course. It's the theory that culture can be profitably viewed from a Darwinian standpoint, but treating it in a very similar fashion to biology: only instead of genes built of nucleic acid, its memes, built out of synapses. The environment memes replicate in is the human brain; the method they use to replicate themselves is imitation. Every time someone imitates someone else, whether by copying someone's haircut, by humming a tune, or by repeating an opinion or a piece of news, they're spreading a meme. This illustrates one of the primary differences between memes and genes: whereas genes are restricted to vertical transmission (parent to child) memes can also use horizontal transmission, ie transmission between unrelated individuals.
Of course, throughout the vast majority of human history this horizontal transmission didn't actually make much difference. That's not to say it didn't give us a huge competitive advantage over other animals: by enabling technology, and rapid behavioural adaptations - in essence a form of evolution that operates orders of magnitude faster than sexual reproduction, the previous evolutionary record-setter - it made us the most versatile species the world has ever seen. What I mean by 'not making much difference' is that, since humans tended to live in small homogeneous groups, they tended to get the vast majority of their memes from their parents. There was limited memetic transmission within the tribe (though very little as members of the tribe would have virtually identical memeplexes), and probably a tiny trickle between (for instance, a hunter seeing a slightly better-made bow, or a gatherer cottoning on to the idea of hitching her baby up in a harness so her hands stayed free for more gathering) but for the most part memes followed exactly the same vector as genes. Their interests were thus almost (though not quite) perfectly aligned.
Modern societies, of course, are quite different. We have books, newspapers, radio, television, and the internet. The majority of the species now lives in vast metropolises, which in turn participate in a globally linked economy consisting of literally billions of people. We're in constant contact with huge groups of people, spending large amounts of time talking, reading, writing and (for some, and more every day) blogging. If a meme wants to spread in this environment, it'll do a lot better if it's geared for horizontal transmission.
One of the predictions of memetics is that the interests of memes and genes will diverge more or less proportionately to the number of horizontal vectors available to the memes. Put another way, people who read a lot have less babies. After all, spreading memes takes time and energy. Child-rearing uses both intensively. From a meme's point of view, any time spent raising a child - even if you spend part of that time passing said meme onto the child - is wasted time. You could, after all, be out there with an active social life, talking to lots of people, and spreading that meme to a lot more than just one tiny little brain. Logically, in such a world, memes that discourage child-rearing (at the expense of meme-spreading) will tend to prosper, as carriers of those memes will have more time and energy to spread them. The logical result of this is a world full of people who don't have kids (or if they do, have the bare minimum: after all, if you do have children, it makes good memetic sense to only have one, as you can maximize the resources expended on the child and thus best prepare them for a life spreading lots and lots of memes.)
If you look at the world, one of the interesting things you notice is that societies really do seem to see their birthrates fall more or less in proportion to how connected they are. The more satellite TVs, computers, and books, the less babies. You also notice that some societies seem to sense this, instinctively, and try to stop it: look at the Taliban, banning every possible vector by which foreign memes might enter their society, or for that matter the recent identical actions of the Islamic Courts Union in Mogadishu. Iran and Saudi Arabia have both attempted similar measures, though with a great deal less success.
Now obviously this is one of those situations that can't last forever. If the birthrate crashes and stays low, eventually the population follows. A falling population is of no benefit to memes; like an epidemic that kills its hosts, being too virulent screws you over in the long run. It's likely that the memeplexes that survive and thrive in the twenty-first century and beyond will be the ones that consciously ensure a high, or at least replacement-level, birth-rate. I'll discuss what some of those might be in the next post.