"Leftists" don't think the correlations between results in IQ tests and many other things (such as memory, income and reflexes) exist?
* IQ causes income
* Income causes IQ
* Some unknown thing causes both income and IQ
* The correlation is spurious; IQ and income are unrelated
Evidence strongly suggests that the third bullet point is true; socioeconomic class causes both income and IQ. Richer people living in nicer neighborhoods both have better opportunities for income, and also better opportunities for education; education causes IQ. This is why redlining is brought up so often as a root cause of so many of the disparities in quality of life; redlining deepened socioeconomic divides.
Adoption studies might also be informative.