MZ twins correlate at ~.85 for IQ versus .55 for DZ twins, giving heritability estimates of 50–80% in adults, with heritability rising from childhood to adulthood.

Evidence

Twin Studies and IQ: Heritability of Intelligence From Childhood to Adulthood (Cogn-IQ.org)

corroborated: 2 independent passages support this claim
XREF: connects to: Intelligence is malleable and shaped by environment, not fixed by heredity — Harriet A. Washington, A Terrible Thing to Waste, 54; With heritability well below 1.00, IQ must be subject to substantial environmental influences, though genetic influence increases with age while shared family environment effects shrink by late adolescence. — Intelligence: Knowns and Unknowns; Twin and adoption studies estimate genetic contribution to intelligence at 50–70% heritability, though molecular GWAS estimates run lower. — Twin Studies and the Heritability of IQ (+1 more)

Twin and adoption studies estimate genetic contribution to intelligence at 50–70% heritability, though molecular GWAS estimates run lower. (similarity 0.90)
Twin studies attribute about seventy percent of intelligence variance to genetics (similarity 0.83)
With heritability well below 1.00, IQ must be subject to substantial environmental influences, though genetic influence increases with age while shared family environment effects shrink by late… (similarity 0.81)
With heritability well below 1.00, IQ must be subject to substantial environmental influences, though genetic influence increases with age while shared family environment effects shrink by late adolescence. (similarity 0.81)
Shared home environment has almost no effect on intelligence variance. (similarity 0.80)
High heritability of individual differences and large environmental effects on population-level IQ are mathematically compatible under a reciprocal gene-environment coupling model (Dickens & Flynn). (similarity 0.78)