Influence of early physical conditions on the development of intelligence, with special reference to the analysis of monozygotic twins

Abstract

The twin study method is deemed to be the most reliable one in order to study the hereditary and environmental influences upon the development of intelligence. Among the various conditions the problem of early physical conditions was this time taken up, using individual intelligence test scores obtained from 269 pairs of monozygotic (MZ) twins and 64 pairs of same-sexed dizygotic (DZ) twins, all twelve years of age. Among the results obtained, the statistically significant ones were as follows:I. The analysis of the physical conditions compared between MZ and DZ twins indicated that MZ twins showed much more similar birth weights, a shorter time interval in birth, and less difference in the initiation of walking than DZ twins. As to MZ twins with a great difference in the start of walking, it was found that the first-born, or the heavier twin began to walk earlier than the other twin, while this relationship could not be seen in DZ twins.II. The view of many previous researchers that MZ twins as a whole show higher degree of conformity in the development of intelligence than DZ twins was not supported by all cases of our MZ twins, and in some MZ twins a great difference of intelligence was found. This difference was considered having been caused by the environmental factors.III. Those MZ twins who showed a great intra-pair difference in their biological conditions at birth were selected. The comparative analysis of the data indicated that1) as to pairs with a great intra-pair difference in birth weight, the heavier twin showed the higher intelligence score, 2) as to pairs with a long intra-pair interval in birth, the first born showed the higher score, 3) when a twin was born with asphyxia neonatorum, he showed a lower score, and4) those twins who showed a great difference in intelligence scores also showed a difference in their initiation of walking and/or talking.Although there was a tendency that MZ twins were born with much more similar physical conditions than DZ twin, there was a great intra-pair difference observed in the development of intelligence at the age of twelve if MZ twins passed through different physical conditions at birth. A definite relationship between the early physical conditions and the intelligence development was thus illustrated, that is, the MZ twin born with superior conditions generally showed superiority in the development of intelligence.