Genes Take the Lead at Ten

Six Studies, Eleven Thousand Twins, Four Countries

In 2010, a team led by Claire Haworth (King's College London) pooled raw general-intelligence data from eleven thousand twin pairs across six independent studies — three from the United States, one each from the United Kingdom, Australia, and the Netherlands — and split them into three age groups, with mean ages of nine, twelve, and seventeen. The result was published in Molecular Psychiatry.

The twin method separates three sources of the intelligence differences between people. Identical (monozygotic) twins share all of their genes; fraternal (dizygotic) twins share, on average, half of the genes that vary between people, the same as ordinary siblings. By comparing how similar twins of each kind are to one another, at each age, researchers calculate three shares that add up to one hundred percent: how much of the difference between children traces to genes, how much to shared family environment — everything siblings from the same house have in common — and how much to experiences unique to each child, which siblings don't share.

By age nine, genes already explain 41% of intelligence differences, and family only 33%.

By age twelve, the split is 55% genes to 28% family. By seventeen, 66% to 16%. Genes aren't just gaining ground — they're gaining exactly the ground family and unique experience are losing. Thomas Bouchard, who named this pattern the "Wilson Effect" after Ronald Wilson, the researcher who first documented it, places the exact crossing point around age ten, regardless of whether the data comes from twin studies or from studies of adopted siblings raised together with no genetic relationship at all.

One Age, Three Shares

Move the slider. Each age shows how the cited data splits one hundred percent of the intelligence differences between children of that age.

Separate studies of Dutch twins (Boomsma et al., 2002) put shared family environment at 55% around age five — more than one and a half times the age-nine figure above. The exact crossing point between genes and family most likely falls somewhere between five and nine; the chart starts at nine because that's the youngest age at which Haworth et al. reported all three shares from the same method, on the same sample.

Why Genes Rise Instead of Fade

One well-supported explanation is that as children grow, they increasingly choose what they read, who they befriend, what activities they seek out and which they avoid. Psychologists call this mechanism gene-environment correlation: a child with a genetic pull toward verbal curiosity will seek out books and conversations that exercise exactly that pull, regardless of how many or how few books sit on the shelf at home. Every year, the child builds more of their own environment instead of simply inheriting one from their parents — and that chosen environment, in turn, reflects the child's genes. Statistically, the effect shows up as a rising genetic share, though the real cause is an interaction, not a competition.

Shared family environment doesn't become unimportant to the child — it stays practically significant, especially while the child is young and depends on the decisions adults make. What the data show is something narrower: that the difference between families matters less and less for the differences between children, because siblings who grow up in different households end up, as adults, resembling each other almost as much as siblings raised under the same roof. Ronald Wilson himself, who first described this pattern in the 1970s, insisted that the finding doesn't reduce parents to irrelevance — it relocates their role, from determining a child's trajectory to supporting the potential already there.