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

Verbal and inductive test results have been collected for a group of male twins in grade 5 at 12 years of age and at enrollment to military service at 18 years of age. MZ twin pairs tend to get progressively more concordant for both verbal and inductive ability from age 12 to 18. DZ twins, on the other hand, get progressively more concordant for inductive ability, while they tend to get less concordant for verbal ability. The results are interpreted with reference to a model taking heredity-environment interaction into account. The discordant trend found when comparing intra-pair similarity in verbal ability for MZ and DZ twins thus seems to indicate the presence of interactional and correlational effects. For inductive ability, however, the difference between within-pair correlations for MZ and DZ twins tends to be of the same magnitude at both 12 and 18 years 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 two tests and possible explanations to the increase from age 12 to 18 in both MZ and DZ within-pair similarity for inductive test scores are discussed.