Genetic and environmental effects on the covariance structure of the Norwegian army ability tests: A study of twins

The ability scores from all complete twin pairs recruited to the compulsory Norwegian military service during the period 1950–1954 were analysed, using a multivariate design. Three separate scores were obtained from the Army tests: general ability, technical comprehension and arithmetical skills. A chi- square model test and a maximum likelihood estimation were performed by the use of LISREL computer program. The total genetic variances varied from 40% to 66% in the three subttests. The environmental within family variances and the environmental between family variances were about equally large. The intra-correlations between the subtests were generally high, and the major part of the genetic variance was common, but specific genetic effects and specific environmental within family effects both explained about 20% of the total variances in technical comprehension and in arithmetical skills. Nearly no variance was specific for general ability. All the environmental between family variance was common for the three subtests.