A Twin Study into the Genetic and Environmental Influences on Academic Performance in Science in nine-year-old Boys and Girls

Abstract

We investigated for the first time the genetic and environmental aetiology behind scientific achievement in primary school children, with a special focus on possible aetiological differences for boys and girls. For a representative community sample of 2,602 twin pairs assessed at age nine years, scientific achievement in school was rated by teachers based on National Curriculum criteria in three domains: Scientific Enquiry, Life Processes, and Physical Processes. Results indicate that genetic influences account for over 60% of the variance in scientific achievement, with environmental influences accounting for the remaining variance. Environmental influences were mainly of the non-shared variety, suggesting that children from the same family experience school environments differently. An analysis of sex differences considering differences in means, variances, and aetiology of individual differences found only differences in variance between the sexes, with boys showing greater variance in performance than girls.

Figure 1

Basic Twin Model Note: A = additive genetic influence; C = shared environment influence; E = non-shared environment influence; path coefficients a, c, and e = effects of A, C, and E on a trait; rMZ = monozygotic correlation; rDZ = dizygotic correlation. Figure shows the path diagram that represents both the scalar model and the null model. The scalar and null models of the sex-limitation design reduce to the basic twin model shown here. This model estimates three parameters: a, c, and e for the sexes combined. In the scalar model, variance differences between the sexes are allowed. In the null model, the variances are equated across the sexes (this is not shown in the path diagram).

Figure 2

Full Sex-Limitation Model and Nested Models Full sex-limitation model and nested models. Note: A = additive genetic influence; C = shared environment influence; E = non-shared environment influence; path coefficients a, c, and e = effects of A, C, and E on a trait, with subscript “m” for males and “f” for females; rg = genetic correlation between same-sex twins, which is fixed at 1.0 for MZ twins and 0.5 for DZ twins; rc = shared environment correlation between same-sex twins, which is fixed at 1.0 for MZ and DZ twins; rgO = genetic correlation between opposite-sex twins; rcO = shared environment correlation between opposite-sex twins. Opposite-sex twins are represented as twin 1 male and twin 2 female, and are linked by rgO and rcO. (a) Full sex limitation model. This model estimates seven parameters: am, cm, em, af, cf, ef, and rgO or rcO. This model allows qualitative sex differences by allowing the genetic and shared environmental correlations (rgO and rcO) between opposite-sex twins to vary. The model also allows quantitative sex differences by allowing the ACE parameters for males and females (am, cm, em, af, cf, ef) to differ. Variances differences between the sexes are allowed (this cannot be seen in the path diagram). (b) Common effects model, which is nested in the full sex-limitation model. The common effects model constrains the genetic and shared environmental correlations between opposite-sex twins (rgO and rcO) to be 0.5 and 1.0, respectively (thereby treating the opposite-sex twins in the same way as the same-sex DZ twins who have a genetic correlation of 0.5 and a shared environmental correlation of 1.0). This model estimates six parameters (am, cm, em, af, cf, ef) and allows variance differences between the sexes. Please see Figure 1 for the path diagram of the scalar and null models.