The Strong Vocational Blank inventory was administered to 43 pairs of identical and 34 pairs of fraternal twins in high schools in the Metropolitan Detroit area, and scores obtained on 44 vocational preference scales and 3 personality scales. F tests of the ratio between fraternal and identical within twin pair variances were significant beyond the .01 level of significance for the scales for Physicist, Mathematician and Osteopath while 9 more scales had F-ratios significant beyond the .05 level; these were for the scales of Dentist, Personnel Director, Veterinarian, Sales Manager, Aviator, Chemist, Public Administrator, Engineer and for the scale of Interest Maturity. Thus a total of 12 scales had F-ratios indicative of some hereditary contribution to the within pair variance of the twins in this study.
A comparison with the results published by Carter in 1932 was possible for 21 scales, by computing intraclass correlations for the present data. From these, as well as from the intraclass correlations reported by Carter, h2 values were computed and compared. There was stronger evidence for hereditary components in the earlier study, and the rank order correlation for the h2 values in the two studies was only .16, although in both studies it was found that the “Science” group of interest scales had the highest “average” h2 value (computed from average intraclass correlations, after Z-transformation).
These results are interpreted as indicative of a small but persistent contribution of hereditary components to the vocational choices reflected in scores on the Strong Vocational Preference Blank, particularly for scientific careers.
The use of the male form for the female twins in this study seems partly justified by the presence of equally often high mean values on scales and higher heritability estimates on some scales compared with the boys.