Life after death - the Second World War and the Baby Boom

For the past 200-odd years, the birth rate in Western industrialized countries has declined, in line with the slow change from a rural to an urban economy, the basic explanation being that large families are necessary and useful on the farm, but not in cities. If we generalize even further, one could say that birth rate declines as the population increases. 

If this explanation is correct, and I think it is, why was there are a Baby Boom? According to Marvin Harris, author of "America Now" (later republished as "Why Nothing Works"), the main cause was also economic (namely the opening up of new markets after the Second World War). I do not dispute that this will have had an effect, but I think that psychological motives were at least as important. 

To me, the gradual decline of the birth rate was a triumph of reason over instinct. The first goal of life in general is to have offspring. Nature does of course have mechanisms to counteract the effects of overpopulation, but as far as I know, only humans are in a position to not only predict the long term effects, but consciously and willingly do something about it. But when we do, we also create a tension between what our bodies were designed to do, and what our brains tell us is best for us in the longer term. 

Combine this idea with the well-know effect that danger has on our impulse to procreate (the link between eros and thanatos), and add a threshold effect, and you have the Baby Boom: after the well-publicized trauma of the Second World War, a lot of people, and presumably especially the victors, found it much harder to suppress the urge to celebrate their survival by having more children. And voila! like a dam breaking, instinct wins out over reason. 

Addendum: this may sound depressing - "how can we ever progress?", you might wonder - but that weighs less on my mind than the irritating suggestion that sheer willpower can overcome genetically encoded natural disposition. A key flaw in the otherwise very entertaining animation film Madagascar is when the zoo-raised lion, in the wild and almost overcome with hunger, succeeds in suppressing  his natural urge to eat his friend the zebra. Obviously, the film is really about humans, not animals, but it is still a ludicrous idea, as I have felt obliged to explain to my children.