Intelligence is understood to be 50% genetically inherited. The reality is that heritability only grows more potent as we age. I’ll start by examining the evidence for this via twin studies.

The 1983 Louisville Twin Study first suggested that intelligence heritability increases through a child’s development. The study included 500 pairs of twins, each assessed 14 times from infancy to adolescence. Identical twins became more similar for intelligence, with correlations increasing from 0.75 to 0.85. Fraternal twins became less similar, decreasing from 0.65 to 0.55. Scientists use the difference between identical and fraternal twins to estimate heritability in studies. Therefore, this pattern of results suggested increased heritability, from about 20% in infancy to about 60% in adolescence.

The sample size of 500 twins was not particularly large. Convincing confirmation of this finding came from the 1985 Colorado Adoption Project.

Correlations between the intelligence of non-adoptive parents and their children increase. Studies show a coefficient of 0.1 in infancy, 0.2 in childhood, and 0.3 in adulthood. Startingly, this was also true for adopted children and their biological parents, whom the children hadn’t seen since the first few days of their life. By sixteen years old, the correlation between adopted children and their parents was the same between non-adopted children and their parents. Meanwhile, the correlation between adopted children and their adopted parents hovered near zero.

Increasing heritability received further support as a hypothesis by the 2010 consortium of twin studies that brought together data on intelligence for 11,000 pairs of twins from four countries. The studies found that the heritability of intelligence increased significantly through childhood (40%), adolescence (55%), and young adulthood (65%).

By sixty-five, studies have shown that heritability reaches about 80%. When we see “50%” heritability of intelligence in media, this is just the lifetime average across all studies.