I've heard dozens of explanations for this seemingly biologically insane behavior, but none of them made sense to me. It's been in the back of my mind for over twenty years.
Now the paradox may be heading towards resolution:
BBC NEWS | Health | Large families 'bad for parents' : "The researchers assert a strong correlation between number of children and maternal mortality. If true this would help understand the apparent wealth paradox. A caveat however, I have little faith left in case control studies. The only reason this one has any persuasive power is that it fits with what we see in other animals.
...They add the findings also suggest why women now tend to have fewer children.
'If women have generally incurred greater fitness costs of reproduction, this could explain why they generally prefer fewer offspring than their husbands and reduce their fertility when they obtain more reproductive autonomy.'
It's probably not only the direct effect of bearing children -- the paternal mortality also rises ...