The best reason to have lots of children - ever!
Only an economist (I think) could come up with an argument like this:
Basic microeconomics recommends a simple strategy. Have the number of children that maximizes average utility over your whole lifespan. When you are 30, you might feel like two children is plenty. But once you are 60, you are more likely to prefer ten sons and daughters to keep you company and keep the grandkids coming. A perfectly selfish and perfectly foresighted economic agent would strike a balance between these two states. For example, he might have four kids total - two too many at 30, six too few at 60.
Trust me - you'll thank me later. Your third child ought to thank me too, but we all know better than to expect gratitude from the young. Now all you have to do is convince your spouse!
There is a certain sense to this, I suppose, and I always love to see logic applied to issues that are generally much more in the realm of emotion. Not because I think it's the way it should be, but because I think the results are often quite amusing. The main thing the author forgets in this (and as a commenter mentioned) is that it doesn't account for grandchildren. Perhaps it would be best to have a couple kids early, so that you could be healthy and active when your kids have children - and you could then have two generations to care about (and for!) you.
This is a post from comes from EconLog, via Marginal Revolution. Marginal Revolution is a great blog, by the way - mainly written by two economists with frequent guest bloggers. They post on all sorts of great (and very interesting) stuff.