Why Two Strangers Raised in the Same House End Up Nothing Alike

Two identical twins, separated at birth and raised by families that never knew each other, end up about as alike in IQ as two siblings who grew up under the same roof. And two strangers, with no genetic relationship, adopted as infants into the same family, end up in adulthood barely alike in IQ at all.

It sounds like the order got flipped.

It didn’t.

And it’s exactly that counterintuitive cross-section of data that has made twin studies one of the most cited tools for understanding where the g factor comes from.

The research design: comparing what’s shared and what isn’t

Psychologist Arthur Jensen explains in The g Factor (1998) why certain pairs of people turn out to be especially valuable to science:

• Monozygotic twins (identical) share 100% of their genes.
• Dizygotic twins (fraternal) share, on average, 50%, like any pair of siblings.
• Adopted individuals, raised alongside genetically unrelated siblings, share the family environment but not the genes.

By crossing these groups (reared together, reared apart, genetically related or not), it becomes possible to estimate, at the population level, what proportion of IQ differences between people tends to be explained by genetics, and what proportion by the shared family environment.

Numbers that don’t match intuition

Jensen documents the average correlations, weighted by the number of pairs studied across dozens of separate investigations. These are the most revealing:

• Identical twins reared together: correlation of +0.86.
• Identical twins reared apart: correlation of +0.75.
• Fraternal twins reared together: correlation of +0.60.
• Ordinary full siblings reared together: correlation of +0.49.
• Genetically unrelated people reared together from infancy: correlation of +0.25 in childhood, and -0.01 (essentially zero) in adulthood.

Read that twice if you need to: two identical twins who never shared a house end up about as alike in IQ (+0.75) as two ordinary biological siblings who did grow up together (+0.49). And two people with no genetic relationship at all, who did share a house, parents, and a routine from infancy, end up as adults with essentially no measurable correlation between them.

What this pattern suggests about the role of the family

This calls for some care, because it’s easy to draw a more simplistic conclusion than the data allows.

What this pattern suggests isn’t that the family home is irrelevant. It suggests something more specific: its weight on IQ resemblance between siblings seems to shrink with age.

• In childhood, even genetically unrelated children raised together show some resemblance in IQ (+0.25).
• That resemblance drops to essentially zero in adulthood.
• At the same time, heritability (the share of variation explained by genetics) tends to increase from childhood into adulthood, according to Jensen.

One way to think about it: in childhood, the shared environment (the same house, the same routine, often the same school) leaves a temporary imprint on cognitive resemblance between siblings. Over time, each adult seems to “find” their own level, more aligned with their own genetics than with the house they grew up in.

What you can’t take from this, tempting as it is

This kind of data invites bolder headlines than it deserves, so it’s worth being clear about the limits:

• These are population averages, calculated across thousands of pairs in dozens of studies. They don’t predict anything about a specific family or person.
• Jensen himself flags an important caveat: twins reared “apart” often still share some prenatal environment and the first weeks or months of life, which can slightly inflate that +0.75 correlation. The estimate is solid, but not perfect.
• None of this says anything about personality, values, emotional wellbeing, or the quality of the family bond, dimensions these IQ-focused studies don’t measure at all.

What this cross-section of data does leave behind is an idea worth sitting with: how alike two people who grew up under the same roof turn out to be says, surprisingly, less than it seems about their future cognitive resemblance. And how alike two people who never shared a single dinner together turn out to be can say quite a bit more.

If you’re curious about another equally counterintuitive finding about the g factor, the test that predicted road deaths better than medical history explains a large-scale study on accident rates that reached an equally surprising conclusion.

Sources

• Jensen, A. R. (1998). The g Factor: The Science of Mental Ability. Praeger.
• Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota study of twins reared apart. Science, 250, 223-228.
• Bouchard, T. J., Jr., & McGue, M. (1981). Familial studies of intelligence: A review. Science, 212, 1055-1059.