So, what probably happens is that women at the bottom of the social heap in the United States, having poor health care, high stressed lives, crime, drugs and all these kind of problems, probably have more spontaneous abortions, therefore the sex ratio  away from males towards females, whereas women at the top of the social scale, low stress lives, good health care, better maternity erm medicine, stuff like that, retain more foetuses, therefore you'd expect them to have more males, and this is what seems to happen.