Similarities and differences in the physical, behavioural and intellectual characteristics of monozygotic and dizygotic twins

Abstract

This study investigated the influence of heredity and environment on children's development through the Classical Twin Method. A sample of 15 pairs of monozygotic and 15 pairs of dizygotic twins aged between five and eleven years and attending ten state primary schools were assessed on physical, behavioural and intellectual characteristics. Quantitative data were collected on similarities and differences in the physical characteristics, behavioural traits and ability and achievement of each pair. Physical and behaviour (Measures of Closeness; Telepathy; Similarity/Difference in tastes; habits; Drawing and Handwriting) characteristics were collected through observations of each twin and questionnaires were administered to each twin and their teacher. Each twin was assessed on six scales of the British Ability Scales (Matrices, Similarities, Immediate and Delayed Visual Recall of Digits and English Word-Reading) and on the Maltese Word-Reading Test (Bartolo, 1988). A correlational analysis using the Bio Medical Data Package (1990) was carried out for all data except for Drawing for the MZ and DZ groups. The results showed that the results obtained were similar to those reported in the literature, i.e. MZ twins' characteristics correlated significantly more highly on all measures. Moreover, this finding applied in descending order of strength to physical characteristics (and to height more than weight), ability, achievement, behaviour, drawing and hand writing. It was concluded that environment has a greater influence on the development of persona lity and achievement than on the other characteristics. The study also suggests the need for a wider spectrum of ability and achievement measures to determine more specific environmental influence.