From the article:
He has also noted that the IQs of the children's mothers and fathers are unknown, and that white and black G.I.'s in Germany were not equally representative of their respective populations, since about 30 percent of blacks, compared to about 3 percent of whites, failed the preinduction mental test and were not admitted into the armed forces. He further argues that the selective preferences of the German women with regard to sexual partners may have influenced the results in an unknown manner.