GCSE grade means (standard deviations).

* to G. N = sample size after exclusions (individuals); MZ = monozygotic; DZ = dizygotic; m = male; f = female; os = opposite sex. ANOVA performed (on cleaned, normality-transformed data from one randomly-selected twin per pair) to test effects of sex and zygosity: results = F statistic; * = p2 = proportion of variance explained by sex, zygosity and their interaction. All variables except for mathematics are composites. Scores for composite means and mathematics GCSE have a maximum of 11 and a minimum of 4, representing grades A