In the past, smarter people had better odds at keeping their legs closed to avoid deadly STDs ( monogamy ), and a stable couple keeps enough of their kids alive to help around the farm to have happy, balanced lives.

They had a big medium term incentive ( help on the farm ) and a long term one to have people care for them in old age, often the one or two kids that stayed single among the 4+ kids who survived infancy.

I saw this in many families in my grandparents generation. The forever single ones ( either socially anxious ou closet gays ) maintained the family farm, cared for their old parents and sometimes watched over nieces and nefews, one of which eventually getting the farm.

Now those incentives are long, long gone, so smart people have it in their interest to not have kids, leaving a sense of duty or future fear of loneliness as motivations to reproduce. So 0, 1 or 2 kids, because more gets in the way of all the distractions of modern life people now focus on.

While stupid people who have no sense of delaying gratification are the ones having more kids ( when they don't abort them all ).