Perhaps someone has posted on this before. GSS allows one to select multiple races. I was able to find 41 self identifying mixed Black-and-White individuals (and five observer identified individuals). Conforming to the prediction of a Black-White hereditarian hypothesis, the (self-identified) mixed race individuals fell in between the parental populations. The Black-White difference in this sample was 0.66 SD and the White-mixed race difference was 0.20 SD or 30% of the parental population difference.
I have now identified four nationally representative samples which bear this prediction out: National Longitudinal Survey of Youth (N=88), Longitudinal Survey of Adolescent Health (ADD Health) (N=128), Longitudinal Collaborative Study (N=129), and General Social Survey (N=41). I’ll leave it for others to investigate if the hereditarian hybrid prediction holds for other populations in the GSS sample.