Although thought to be substantial, within-person variability in school grades has not been systematically studied. Here we analysed data from the Twins Early Development Study (TEDS; Nmax = 11,132) to describe within-person variability across grades in English, maths, and science from age 7 to 16 years. We found that within-person grade variability was largely unstable across subjects and ages. Within-person grade variability at age 16 was not associated with any of 15 variables that typically explain between-person differences in school performance (e.g. IQ, socioeconomic status, and personality traits). Also, within-person grade variability did not predict later educational outcomes at ages 18 and 21. Our findings suggest that within-person grade variability is an observable, but not meaningful psychological construct. We conclude that understanding the causes and consequences of within-person grade variability is of limited epistemological value.

We tested the relative influence of mothers’ and fathers’ education on children’s differences in intelligence (IQ) across five UK population cohort studies (total N = 48,175) with children born in 1946, 1958, 1970, 1994-96, and 2000. Associations between mothers’ and fathers’ education with children’s IQ ranged from r = .38 (CI 95% .36 - .41) to r = .19 (CI 95% .17-.20) across generations. Fathers’ education was more strongly associated with children’s IQ in earlier cohorts but later, mothers’ education became more influential. Negative interactions between mothers’ and fathers’ education, suggesting compensatory effects, influenced children’s IQ only in the earlier cohorts significantly. We conclude parents’ education has continuously influenced children’s IQ with medium to large effect sizes across generations.