While this is true, it's also true that significantly fewer families can do it now than could in 1970.
It's also true that the most predictable factor when it comes to student performance in school is household income.
It's also true that the most predictable factor when it comes to student performance in school is household income.
Assuming you mean predictive rather than predictable, the most predictive factor from what I've seen is parental educational attainment, not income or (to respond to a sibling comment) IQ.