Adoption studies including identical twins adopted apart show genetic influence on intelligence. 

Plomin conducted a multivariate genetic study in 1986 on 7,000 twin pairs who were assessed longitudinally using parent-administered tests.  Plomin found thst about half of the phenotypic correlation between the home and children’s intelligence is inherited genetically.  Plomins twin study findings indicates that heritability is lower in infancy (about 20%) then in middle childhood (about 40%) at 2, 3, and 4 years.  Plomin showed that heritability of intelligence increases during development.  Plomin showed how heritability increases from about 40% in childhood to about 60% in early adulthood to 80% in later life. 

Another twin study done by Pedersen, Plomin, Nesselroade and McClearn in 1992 shown that twins at 60 years of age, whether they were raised together or apart, indicated a heritability estimate of 80% .  Which is the highest heritability ever reported.