This study would be fascinating enough with those findings. But these authors also found persistent environmental influences on IQ. Another interesting effect is the passive covariance between genes and environment (.11 at age 15 and .03 at age 32), which can occur when the parent’s genes impact the environment that a child experiences.
Genes, environment, and developed traits are involved in an intricate dance where each can influence the other across generations. The debate isn’t “nature vs. nurture” any more. The question is how nature and nurture interact.
The persistent environmental effect is crucial and often overlooked in discussions that overemphasize heritability. Even at age 32, adopted children still showed environmental benefits from their adoptive families, and the correlation between adoptive parent IQ and child IQ (r = .06 at follow-up 3) suggests lasting influence. The fact that non-shared environment accounts for 50-64% of variance in the follow-up data indicates that individual experiences matter substantially throughout life.
@M.Evanta Exactly - non-shared environment includes any environmental influences that differ between siblings in the same family: different peer groups, teachers, illness, birth order effects, differential parental treatment, and random life experiences. The finding that these account for 50%+ of variance suggests that unique individual experiences have lasting impacts on cognitive development, even when controlling for genetics and shared family environment.
Yes, nature and nurture interact, but the interaction here shows genes winning over time. Heritability rises, adoptive family effects vanish, and the environmental variance is mostly non-shared. The practical takeaway isn’t that interventions work, it’s that genetic predispositions increasingly shape the environments people create for themselves.
Non-shared environment isn’t the same as “meaningful experiences.” It’s a catch-all category that includes measurement error and random noise, not just life experiences that matter. You can’t conclude that individual experiences have lasting impact just because this category is large.
The heritability increase from .32 to .42 is striking. What explains the environmental effect dropping from 3.48 to 2.83 points between ages 15 and 32?
It’s more likely due to fadeout. When adopted children become adults, they go out from their adoptive family environment and increasingly create their own circumstances, which tend to correlate with their genetic tendencies.
That’s correct about the fading shared family environment, but it only accounts for a tiny fraction of the whole. The most important environmental takeaway is the massive 50% chunk of non-shared environment (E) which represents unique, non-familial life experiences that shape adult IQ, like specific teachers, peer groups, jobs, or even random events. So, the practical takeaway isn’t that genes are the only influence, but that individual-specific environmental events are just as critical as genetics in determining adult IQ.
Even if we assume half of the Non-Shared component is measurement error and random noise, the remaining 25% is still a larger, more statistically relevant environmental impact than the shared sibling environment (4%) or the parental environment (1%) found in the study. Therefore, the finding that systematic shared family effects are negligible remains a strong conclusion. The bulk of the real environmental variance must reside in the E component, even with its limitations.
If adoption improves a person’s environment by 1 SD, we can expect IQ to increase by 3.48 IQ points (at age 15) or 2.83 IQ points (at age 32).
