There are tons of socioeconomic factors to consider, but I know a lot more fairly average IQ couples with 4 kids instead of two, and I know a lot more smart people who are either childfree or single than parents.
The ability to use birth control properly requires some intelligence.
The needs/career ambitions of a knowledge work career sometimes push aside the desire for a family.
Not trying to be down on people who have lots of kids! There's just tons of factors, some less obvious.
Is it possible that you associate things that correlate with rejection of children with iq? Like ambitions and career - low ambition don't necessary imply stupid. May be that wish for children tamper ambition and career.
The ability to use birth control properly requires some intelligence.
The needs/career ambitions of a knowledge work career sometimes push aside the desire for a family.
Not trying to be down on people who have lots of kids! There's just tons of factors, some less obvious.