We report the first major genetic study of mild mental impairment. From a representative sample of 3,886 twins (1,314 monozygotic, 1,296 dizygotic same-sex, and 1,276 dizygotic opposite-sex), the lowest 5% were selected on a composite measure of verbal and nonverbal abilities assessed by parents when the twins were 2, 3, and 4 years of age. Twin concordances for mild mental impairment were 74% for monozygotic twins, 45% for same-sex, and 36% for opposite-sex dizygotic twins, indicating substantial genetic influence. Model-fitting analyses estimated group heritability as 0.49, which is significantly greater than heritability for the rest of the population in early childhood. These results suggest that mild mental impairment is a good target for neuroscience research on global brain function and dysfunction.