They didn't think it far out enough...

http://www.livescience.com/history/070215_fewer_children.html

A. Children are more expensive.

B. Lifetimes are longer, thus an individual is relatively, from a simple
economic standpoint, more valuable.

C. Tied to the lifetime differences, the number of people who lived through
first birth, then infancy and teenager hood, and so on, is greater and
greater every year. This itself is merely an observation...

D. Mothers are having children later, children are reaching level of social
maturity later (getting into the work force, finishing education and then
starting their family).

E. The time alive per generation (TAG) will not drop as much as the relative
drop in the absolute number of individuals which, from a genetic standpoint,
would seem to say something very important, and that has to do with the
success of the genetics.

F. Due to the lower number of children and the greater economic needs to
develop one properly, those individual children are valued greater as the
relationships from an emotional level would become greater. The individual
child means more emotionally to lose than one of 10, none of whom you were
able to get amazingly close to (other than the eldest or youngest of
children). Each child has greater value, which would again create a greater
value per generation than would be expected with the significant drop off in
absolute numbers.

G. The economic necessity of children is lesser as individuals are able to
better to hedge against their future and are no longer dependent upon
children to support them in their old age.

As we look all the way down in nature we find that the number of young born
continually drops the more valuable the off spring is...why does this
surprise us? Come on now...