Kerry Howley has a fascinating piece in Reason on fertility panics. Let me make a few points I consider obvious: entitlements are not a reason to have more children, natalist panics often have an unpleasant flavor of racism to them, and ever-increasing populations are neither sustainable nor desireable. The transition we are currently facing will be moderately difficult, but as I wrote for The Atlantic, it will not be a disaster, and at the end of the transition, we will still be a wealthy society with a lot to live for.
That said, I think Will Wilkinson goes too far when he says
The way I see it, those obsessed with fertility are people who think the culture they desire cannot possibly win the argument against competing cultures. So, they conclude, it’s down to brute baby-making force: the culture that wins the fertility war wins the culture war. In contrast, I think liberal market culture has such immense, salient rewards (wealth, longevity, happiness, etc.) that it is not only possible to win the argument, but that we are in fact winning it. Of course, part of the winning is dynamist cultural synthesis. So if you’ve got a conservative, zoological view of cultural preservation which fixes on the importance of high-fidelity copying of inessential aspects of a culture’s history (costumes, holidays, rites, cuisine, skin colors etc.), you’re going to have a hard time of it. But if you care about the essential core of liberal modernity, you should be delighted with how things are going. You’ll eat your szechuan taco pizza and you’ll love it.
Cultures don't have "arguments". The most important core beliefs most people have are transmitted not through dialogue, but through inheritance. It is extremely likely that you share the political views of your parents, their religious affiliation, and at a less obvious level, their beliefs about things like what constitutes stealing and lawbreaking, and what are justified evasions of petty laws. My mother is the one who would make us turn around and drive twenty miles back to the store if it turned out we'd forgotten to pay for things, and I now have the same attitude.
The overwhelming evidence is that when it comes to culture, numbers trump ideas. in the successive invasions of England, for example, the locals always won unless they were wiped out; the conquerors assimilated. Or look at America. More people here claim descent from the Irish than the English; numerically, Irish Americans are the largest single-country ethnic group. Yet our culture is much more heavily derived from English Prostentantism than from Irish Catholicism. That's because the earlier waves of assimilated immigrants had adopted the fundamentally British culture of America, and outnumbering the Irish, forced them to assimilate. Successive waves of immigrants have each left their cultural mark, changing (I devoutly believe) us for the better, and also to something that cannot be called British. But the dominant strain remains English. Cultures that "win" the argument in territory outside their own do so by killing, swamping, or removing the previous inhabitants.
Food is awesome, but it is culturally trivial. A little while ago, I said that I thought that liberals underestimated the extent to which the welfare state is spending down cultural capital accumulated in an era before safety nets. Similarly, I think libertarians tend to vastly underestimate the extent to which liberalism and free markets are sustained not by proclaimed belief or legal institutions, but by unobserved cultural norms that are transmitted slowly, if at all. Mexico can see all the things that are better about the US, none of which are particularly difficult to reproduce at the institutional level, but enforcement depends on things like a visceral indignant reaction to policemen who take bribes, rather than an attempt to work the system by developing friends in the police force. Tyler Cowen now believes that returning immigrants are shifting those norms, but we're talking about a process of decades, if not centuries. I deeply enjoy having access to world music, world art, world food--but none of them improves my life as much as living in a country with robust cultural support for individual freedom, democracy, capitalism, and liberalism.
Countries that have a real natalist problem--not America or most of Europe, by the way--are right to worry. If Israel is demographically swamped by the Arabs in the occupied territory (as they will be if they don't reach a two-state solution pretty quickly), the character of the country will change dramatically. You'll still be able to get kosher sausage, but the way the government and civil society work will change dramatically, and in many ways not for the better. Likewise, the Protestants in Northern Ireland were perfectly correct to be concerned that if the Catholics reached a majority, their country would change a great deal.
I don't think natalism is the right way to deal with this problem; I think the answer is to handle immigration in a way that allows the immigrants to be easily assimilated into your culture. To the extent that Europe does face a threat from immigration, it's because their policies flatly discourage assimilation. On the one hand, they lavish welfare benefits on immigrants and often make extensive accomodation to their interest groups; on the other, their employment system discriminates, their culture does not consider immigrants to be real members of society, and when they do attempt to encourage assimilation, they do so through ham-fisted authoritarian measures like the French ban on headscarves. America does very well with a policy of benign neglect: no one's going to force you to assimilate, but you'll have a hell of a hard time staying here unless you do.
I'm enough of a cultural relativist to believe that other cultures have a perfect right not to adopt American values, but enough of a cultural hegemonist to know that I want my country to maintain the dominant American culture. I will be thrilled to have new generations of immigrants contributing their music, their food, their religion, their community life--but I do not want them contributing their ideas about the rule of law, or indeed, what constitutes acceptable behavior in a queue. I think it is possible to achieve this happy balance without having nine kids--and without freaking out at the thought that future Americans will not share my green eyes, snub nose, and near-flourescent albedo.