The inheritance of scholastic abilities in a sample of twins. II. Genetical analysis of examinations results.

1. Examination results for 98 pairs of same-sex and 39 pairs of opposite-sex twins were available for analysis as well as measurements of IQ and height for 47 of the same sex pairs. 2. The choice of subjects taken for examination is not influenced by genetic differences within DZ pairs but seems to be mainly determined by between family cultural differences. 3. Neither the sex difference in opposite-sex pairs nor the difference between first and second born twins in same-sex pairs contributes appreciably to within pair environmental variance. However, same-sex DZ pairs are more often found in different school classes than MZ pairs. 4. No evidence for interaction between genotypes and within-family environments was found for any character. 5. The simple environmental model fails to fit the data for English, French, Geography, Mathematics and IQ whereas the simple genetical model fits. For the remaining subjects it is not possible to exclude either hypothesis. The heritablities of the examination performances are of the same magnitude as that for IQ. 6. Sampling inadequacies are revealed in the data for height but there is evidence for a substantial genetics component of phenotype variation. 7. There is evidence for heterogeneity of gene action in the subjects English and Mathematics supporting the view that there are genetically determined specific abilities acting independently of general intelligence.