identical twins reared together vs apart, their IQs correlate more with their twin's than with the SES of the adoptive family.
high quality studies have found that the additive heritability of IQ is somewhere around 0.8-0.9, that's just totally incompatible with parental SES as the primary upstream cause. it's simply not debatable. and it wasn't debatable ten or twenty or fifty years ago either; it is one of the most well-established facts in human quantitative genetics.
ironically one of the things having a high IQ is bad for is having accurate beliefs about IQ; ask anyone who didn't go to college and odds are they'll correctly tell you that smartness runs in families.
