Stupidity may actually be a factor that leads people to have more kids, so we may see more of it in the future :)

> Evolution only selects for hereditary, biological factors,

That's true. And we really don't know what the full list of such factors are hidden in our genes. Maybe many, maybe just a few. What's for sure, though, IF there are enough genes somewhere in the gene pool that lead people (women in particular) to want to have kids at an elevated rate, those are massively selected for right now.