The laterality effects in 10 symmetrical EEG derivations in twins (20 MZ and 20 DZ pairs with a mean age of 20.5 years) were examined. The quantitative and qualitative analyses gave the following results: (1) cotwins in the MZ and DZ pairs differed particularly in the intensity of asymmetry for EEG parameters — one was more asymmetrical than the other; (2) among the MZ twins there were no “mirror” pairs (opposite asymmetry of the EEG), even where opposite-handedness existed. For example, a right-handed twin had an asymmetrical EEG, while the other, a left-hander, had a symmetrical one; (3) the most asymmetrical EEG was in the temporal derivations showing a more active left hemisphere; and, (4) there was no evidence of genetic influence in the intensity of EEG asymmetry.

Procedures are described for the acquisition and analysis of data in a study of the dental and facial characteristics of South Australian twins. Comparisons of the mesiodistal diameters of maxillary incisors in MZ and DZ twins revealed heterogeneity of total variances and evidence of inequality of mean values for some dimensions between MZ and DZ twins. Previous estimates of heritabilities for tooth size, relying on classical assumptions in twin research, may be exaggerated. A preliminary analysis of facial shape was undertaken using a procedure for shape matching based on a least squares fit of homologous coordinates. There was evidence of mirror-imaging in some MZ twin pairs and differences in facial asymmetry between male and female DZ twins. Future extensions of the study using methods for three-dimensional shape analysis are described.