this section of the study, Hirsch is opposing some straw man version of Nordicism:
The Racialists maintain not only that the Nordics are long-headed, tall people with blue eyes and light hair, but more importantly it is claimed that they are mentally superior to all non-Nordic races.And the results satisfy him:
that intellectual differences although (1) significant, (2) innate and (3) relatively permanent are Natio-Racial in character rather than Racial.Which definitely shows "the Racialists". I did find the differences between pigmentation morphotypes within some national groups interesting (though I don't put undue weight on these results), considering light hair and light eyes have apparently been reported elsewhere as correlates of IQ. You can read the entire monograph (which includes more information on the tests used and answers to some criticisms) online at Harvard's open collection on immigration.
Reference: Hirsch, Nathaniel D. M. A study of natio-racial mental differences. Worcester, Mass. : Clark University, c1926.