One of the more serious explanations of the post-war Baby Boom is economist Richard Easterlin's cohort hypothesis. According to him, children of the Great Depression grew up accustomed to a low standard of living, and when they participated as adults in the economic boom after WWII, they felt like they could afford a large family. And they certainly had large families, averaging almost four children. Keep in mind that this was in spite of a longer-term decline in fertility (one seen long before and after the Baby Boom).
Easterlin explains that Boomers grew up during prosperous times and developed high expectations for their own futures. As they entered adulthood, however, they were confronted with bad ecomomic times, and so the birth rate collapsed.
A good deal of research supports the idea, and it makes me wonder, for those who want to be grandparents someday, if it isn't counterproductive to spoil your kids. If they grow up having everything handed to them, they might expect a lot for themselves as adults and are likely to take out any economic troubles on their potential children. I grew up around small-town people who make their children work for everything, and it makes the kids feel rich when they grow up and get a job that pays so much more than what they are used to having at their disposal.
It make sense that Mexican immigrants have more kids here than they would back home because when you are used to Mexico's wages, working here can make you feel like you hit the jackpot (or at least it used to).