The inheritance of scholastric abilities in a sample of twins. I. Ascertainments of the sample and diagnosis of zygosity

A sample of 149 twin pairs was ascertained by extracting all pairs of candidates with the same surname, birth date and address from the files of a public examination taken by about two thirds of the 15-year-old cohort in South Australia. Of these there were 39 opposite sex pairs and 100 same-sex pairs whose zygosity was determined by postal questionnaire. This diagnosis was checked and found to be accurate by typing 47 pairs for ten independent genetic markers. Several analyses suggest that the ascertained twin samples is fairly representative of the fraction of the cohort who took the examination so there is no evidence to suggest that between families components of variance will be underestimated for this reason.

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.

Phenylthiocarbamide tasting in a sample of twins

1. Twenty-eight pairs of monozygotic and eighteen pairs of dizygotic twins were tested for their ability to taste phenylthiocarbamide (P.T.C.) by the method of Harris & Kalmus (1949). 2. Much greater variance in threshold levels was detected within the DZ than within the MZ pairs and this confirms the genetic origin of most variation in ability to taste this substance. 3. Variance in threshold levels within MZ pairs is of the same magnitude as the variance of reported duplicate measurements on the same individuals. 4. DZ twin pairs were classified as either concordant or discordant in their tasting ability. Variance within concordant DZ pairs is significantly greater than within the MZ pairs and it is shown that this difference can be accounted for in terms of the incomplete dominance of the T allele reported by Kalmus (1958). However, the data do not exclude the possiblity that this greater variation in threshold levels is partly due to multiple alleles or background genetic variation. 5. The frequency of the t allele in Australians of European descent in estimate at 0-52 plus or minus 0-06.

The Montreal Type A Intervention Project: major findings

This article reports a comparison of three short-term treatments (aerobic exercise, cognitive-behavioral stress management, and weight training) in modifying behavioral and cardiovascular reactivity to laboratory psychosocial stressors in healthy Type A men. One hundred seven men completed the treatments and evaluations, 33 in the aerobic exercise group, and 37 each in the cognitive-behavioral stress management and weight-training groups. The stress management group showed significantly greater changes in behavioral reactivity (reductions of 13% to 23% below initial values) than the two physical exercise groups, which did not differ significantly from each other. For physiological reactivity, changes attributable to intervention were trivial for all three treatment groups. The positive finding of reduced behavioral reactivity as a result of the stress management intervention is of potential clinical significance and warrants further exploration. The lack of meaningful reductions in physiological reactivity also requires further exploration in that it raises questions concerning the ability of behavioral treatments in general to modify physiological reactivity, the ability of existing measures to assess accurately changes that are produced and, most fundamental of all, the relevance of physiological reactivity as an outcome measure for treatment efforts with Type As.