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.