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and we can just assume both parents usually work because that's normal. So 100,000 is on the border between "upper middle class" and above.

50K is median HOUSEHOLD income, not median individual income. It includes the incomes of everyone in the household.

Dual-income households are more common than not in two-parent homes, but there are many, many single-parent households, and there are quite a lot of SAHMs and moms who work PT. Contrary to popular belief, SAHM-dom is not the province of the privileged, either.

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We need to be able to talk openly about differences between parenting styles correlated to SES and not automatically blame all differences directly on money.

This is actually talked about quite a bit in the academic literature. It may not make it to the popular press as much. There is certainly a correlation, but it's obviously not a perfect one. Programs like Geoffrey Canada's Harlem Baby College start with pregnant moms to try to rejigger parenting and discipline beliefs with all this in mind.

One thing to consider when thinking about cultural differences in childrearing is that for young men of color especially, life and limb can depend upon being correctly and swiftly responsive to commands from law enforcement figures. Parents therefore may be very interested in teaching children to obey at whatever cost, and to respect authority at whatever cost, a consideration that is unlikely to be a priority for parents of, for instance, wealthy white female children. I read a fascinating study about how different children approach or don't approach teachers for help. Wealthy/UMC children ask for help from teachers much more often and are much more assertive and persistent if they still don't "get it." Poorer children and children of color are less likely to ask for help and will say "okay" even if they don't understand.