This seems to be an objective genetic fact.
That people who are mildly mixed to similar ingroups (I'm SPECIFICALLY talking about shit like: "I'm 1/4 Irish, 1/4 Italian, 1/4 German, 1/4 French") have "hybrid vigor" which makes them genetically superior.
While the exact OPPOSITE group is the Jews (most ingroup marrying) ironically.
Also overly hybrid is ALSO BAD (mulattos, Hispanics etc), it seems that being in the middle is best.