Another way (besides just pure luck) that two below average intelligence people could produce above average intelligence children is this:
A man with (overall) low intelligence is none the less good at math mates with a woman of (overall) low intelligence is none the less good at verbal skills. If their children were lucky, they might get his great math skills and her great verbal skills.
I see this in my own children, where some of my four children lucked out and got the best of both of our strengths, while others more closely mirror me or my wife.
After examining statistics from 27 nations, a group of researchers found the presence of book-lined shelves in the home — and the intellectual environment those volumes reflect — gives children an enormous advantage in school.
“Home library size has a very substantial effect on educational attainment, even adjusting for parents’ education, father’s occupational status and other family background characteristics,” reports the study, recently published in the journal Research in Social Stratification and Mobility.
“Growing up in a home with 500 books would propel a child 3.2 years further in education, on average, than would growing up in a similar home with few or no books.”
I got my Mom’s great verbal skills and my brother got my Dad’s great math skills so much so that colleges were offering him scholarships as a soph. I am a math mutant.