The significance for intelligence of differences in birth‐weight and health within monozygotic twin pairs

Abstract

This paper presents further evidence to demonstrate the existence of intra-uterine effects within the normal range of intelligence. The argument is then extended further to estimate the effects of organic factors in the environment that are also pathogenic for intelligence. The combination of these and intra-uterine effects is found to be a substantial part of the variance associated with environmental factors. Various implications of these factors are discussed.