Intra-pair similarity in IQ of monozygotic and dizygotic male twins at 12 and 18 years of age

Publisher Website
Google Scholar
Abstract
Verbal and inductive test results were collected for a group of male twins at 12 yr of age and at enrollment to military service at 18 yr of age. MZ [monozygotic] twin pairs tend to become progressively more concordant for both verbal and inductive ability from age 12-18. DZ [dizygotic] twins become progressively more concordant for inductive ability and less concordant for verbal ability. Results are interpreted with reference to a model taking heredity-environment interaction into account. The discordant trend found when comparing intrapair similarity in verbal ability for MZ and DZ twins seems to indicate the presence of interactional and correlational effects. For inductive ability the difference between within-pair correlations for MZ and DZ twins tends to be of the same magnitude at 12 and 18 yr of age. Probably this type of test is less differentially influenced by the environments being sampled, at least under present circumstances, when children are not specifically trained to solve the kind of items included in the inductive test. Regression effects for the 2 tests and possible explanations for the increase from age 12-18 in both MZ and DZ within-pair similarity for inductive test scores are discussed.