http://medicine.jrank.org/pages/2465/Intelligence-Family-Twi...
"In twin studies of over 10,000 pairs of twins, monozygotic (genetically identical) twins averaged an 0.85 correlation of "g," whereas for dizygotic (fraternal, like brothers or sisters) same-sex twins the "g" correlations were 0.60. These twin studies suggest that the heritability (genetic effect) accounts for about half of the variance in "g" scores."
https://en.wikipedia.org/wiki/Heritability_of_IQ
Cites this particular twin study: https://www.ncbi.nlm.nih.gov/pubmed/7945151
"The powerful quantitative genetic design of identical and fraternal twins reared apart (112 pairs) and matched twins reared together (111 pairs) was employed to assess the extent of genetic influence on individual differences in cognitive abilities during the last half of the life span. General cognitive ability yielded a heritability estimate of about .80 in two assessments 3 years apart as part of the Swedish Adoption/Twin Study of Aging. This is one of the highest heritabilities reported for a behavioral trait. Across the two ages, average heritabilities are about .60 for verbal tests, .50 for spatial and speed-of-processing tests, and .40 for memory tests."
If we accept the above correlation, there seem to be 2 likely explanations for this, outside of pure coincidence.
One may posit that this relationship is due to some inheritable trait which causes high income earners to consistently produce offspring who make the same sound decisions they made based on intelligence.
If this were true, I would expect to see far more high functioning individuals at the top of the economic pie. Purely observationally, this seems not to be highly correlated. There are lots of exceptions, but high income seems to be associated mostly with a history of high income.
Alternatively, and I believe more likely, this difference can be explained by the difference in access to resources which wealth provides.
If IQ is just a proxy for family wealth, what can or should we do about it? It seems most equitable to simply provide broad access to high quality education to those who can keep pace at a cost (both in terms of money and social friction) approaching zero.
[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4641149/