True grit and genetics: Predicting academic achievement from personality.

4,642 UK 16-year-olds forming 2,321 twin pairs (883 monozygotic, 761 same-sex dizygotic, 677 opposite-sex dizygotic); GCSE grades verified against the National Pupil Database for 7,367 twins

United Kingdom; the Twins Early Development Study, UK-representative, age 16.

The key hereditarian source for this topic. Classical twin design decomposing the variance in grit and the Big Five and in their covariance with GCSE examination grades - an objective national attainment measure rather than a teacher's rating. The authors note this is the first twin analysis of grit.

Grit Perseverance of Effort is 37% heritable and Consistency of Interest 20%; all personality traits fall between 20% and 38%. Shared environment is negligible and NOT SIGNIFICANT for every trait. Phenotypically, Grit Perseverance correlates with GCSE grades at r = 0.17, conscientiousness at 0.24, and Consistency of Interest at 0.06 (not significant). Grit Perseverance and conscientiousness correlate 0.53 phenotypically with a GENETIC correlation of 0.86. The Big Five explain 5.6% of GCSE variance and adding BOTH grit subscales raises this by only 0.5%. Bivariate heritability shows 88% of the grit-GCSE correlation is genetically mediated. The authors state the most limiting finding for any possible intervention - shared environmental influence is negligible, meaning current differences between families and schools explain little variance in the development of grit.

This IS the genetic evidence, subject to the classical twin design's equal-environments assumption.

Unreplicated as a twin study of grit specifically. The pattern it reports - heritability around a third with shared environment at zero - is the standard behaviour-genetic result for personality and is reproduced for self-control by Willems et al. 2019.