[aik]

"No doubt all these social explanations are true as far as they go, but they do not address the deeper question of why people’s psychology should have evolved in a way that makes them want fewer children when they can afford more."

I've always explained this as so (I have no evidence for any of this): 1. People in different social classes have different inherent desires in life. Some people simply want a family and have no other ambitions. 2. Lower class people (or people that have more children on average) have a harder time delaying gratification on average. Delaying gratification is a skill very beneficial when attempting to be successful and is related to the ability to think ahead and visualize the future. Having this ability instills different desires in a person and may detract a person away from parenthood due to truly understanding how much of a commitment children actually are (20+ years of your life, hours a day). Having this ability also helps one realize the difficulty in caring for many children.

[psranga]

My wife works in development. Her theory is insightful and simple: she says mortality from disease (even until age 20) is VERY high for poor people (I know of a poor 10-yr old who died of rabies in India, which is extremely uncommon among the middle-class of India). So they're being rational by having more kids.

Which is why development reduces fertility; people have higher confidence that their brood will survive into adulthood even if it's smaller.