From the New York Times:

"All Brains Are the Same Color"

Richard E. Nisbett

... During World War II, both black and white American soldiers fathered children with German women. Thus some of these children had 100 percent European heritage and some had substantial African heritage. Tested in later childhood, the German children of the white fathers were found to have an average I.Q. of 97, and those of the black fathers had an average of 96.5, a trivial difference.

This Eyferth study was published in 1961 and has been relentlessly cited ever since. And yet, nobody has replicated this 46 year old analysis. It's not as if African-American soldiers stopped fathering half German children so that it can't be done anymore. It's just that nobody seems to want to do it. I wonder why.

By the way, perhaps the best-known son of an African-American soldier and a German woman was Rob Pilatus, one-half of Milli Vanilli, the Grammy Award-winning singers who didn't actually sing.