Society is cognitively stratified: Smart people tend to earn higher educations and higher incomes, and to work in more prestigious occupations. This holds for people from excellent family backgrounds (Utopian Sample) and comparing siblings from the same families!

This is true, meaningful, and the causal relationship runs strongly from IQ to SES, with little independent influence of SES. Just look at how similar the overall result and the within-family results are! But also look at fertility in this table: quite the reverse!