When Yerkes in 1915 published his Point Scale Revision of the Binet Tests, he included among his findings data relating to the social and racial status of children tested. He concludes "that conditions which are in part describable as sociological are correlated with differences in intellectual performance, which may amount to as much as 30 per cent of the total." He says: "In view of this fact...

The data employed in this study consist of a portion of the material gathered for an extensive investigation of identical and fraternal twins. The measurements were made by Professor H. H. Newman (Department of Zoology), Professer F. N. Freeman, Mrs. Bryan Mitchell, and the writer, all of the University of Chicago. A number of articles and monographs based on these data are now in preparation. ...