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Adopt a Child, Drop an Illusion. Effects of adoption on intelligence: 42% Heredity, 8% Environment?

I don’t do policy, but how about this one? In addition to all public policies aimed at getting rid of the achievement gaps between different groups, why not take an intensive approach? Continue with every program which is already under way, but add this one.

Get every child who is under-performing to live permanently from an early age with an adoptive family. This will give the children a deep immersion in all the things that good families can give their own children.

We will carefully select adoptive parents who really want to adopt, many of which will have no children of their own, but very much want to bring up children.

This is an extreme experiment, far more profound than having just a few hours of enriching experience at a nursery. It will affect every waking moment, every spontaneous incident and remark, each instant of togetherness. Every shared experience can be used to teach, train and pass on the insights of wealth and status.

How much will this boost IQ? To give this experiment every chance of showing its effects, we will wait till the children have grown up, and are 30 years of age. By now they should be well-established into their upward trajectories as a consequence of their enriched upbringing.

We do not actually have to do this experiment, many researchers have followed up adoptees.

Here is the most recent study.

  • Genetic and environmental contributions to IQ in adoptive and biological families with 30-year-old offspring. Emily A. Willoughby, Matt McGue, William G. Iacono, James J. Lee. University of Minnesota Twin Cities, Department of Psychology, Minneapolis.
  • Intelligence 88 (2021) 101579

The authors say:

While adoption studies have provided key insights into the influence of the familial environment on IQ scores of adolescents and children, few have followed adopted offspring long past the time spent living in the family home. To improve confidence about the extent to which shared environment exerts enduring effects on IQ, we estimated genetic and environmental effects on adulthood IQ in a unique sample of 486 biological and adoptive families. These families, tested previously on measures of IQ when offspring averaged age 15, were assessed a second time nearly two decades later (M offspring age = 32 years). We estimated the proportions of the variance in IQ attributable to environmentally mediated effects of parental IQs, sibling-specific shared environment, and gene-environment covariance to be 0.01(0.00-0.02], 0.04 [0.00-0.15], and 0.03 [0.00-0.07] respectively; these components jointly accounted for 8% of the IQ variance in adulthood. The heritability was estimated to be 0.42 [0.21-0.64]. Together, these findings provide further evidence for the pre-dominance of genetic influences on adult intelligence over any other systematic source of variation.

While adoption studies have provided key insights into the influence of the familial environment on IQ scores of adolescents and children, few have followed adopted offspring long past the time spent living in the family home.

So, bright parents create bright environments (add 1%), brothers and sisters in that family contribute to that environment (add 4%), and the interaction adds another 3%, thus 8% in all. However, if we look at the confidence limits (in brackets above) the low-ball figures could conceivably be nothing for the environment and 21% for genetics. The high-ball estimates would be 24% for environment, 64% for genetics. An even larger sample of adopted children would reduce those confidence limits.

At the Minnesota Center for Twin and Family Research (MCTFR), the Sibling Interaction and Behavior Study (SIBS) has followed a sample of adoptive and biological Minnesota families for nearly two decades. Initial IQ assessments were conducted when offspring were approximately 15 years of age, and this paper reports on new assessments taken at approximately 30 years of age. At age 15 children will still be under family influence, by 30 they will have established their own lives.

A recent development which transforms adoption research is to include entirely gene based polygenic risk scores. These are the new kids on the block, which provide the first approximation to a pure predictive genetic score. At the moment they are not able to capture all the effects which we know are due to heredity, as shown in twin research, but they are steadily increasing in power.

Our polygenic scores for educational attainment provide what is, at moment of writing, the largest R 2 estimates for any cognitive phenotype (0.113 for Total IQ; 0.154 for Verbal IQ). The particularly high predictive validity for PGS EA for verbal IQ is perhaps to be expected given that verbal IQ correlates more strongly than other IQ subscales with educational attainment, particularly for parents. These scores also enable a unique test for the so-called “placement effect,” wherein adoptees (typically twins reared apart) are thought by some skeptics to resemble their adoptive parents prior to placement, thus biasing biometrical estimates. By demonstrating a total lack of evidence (p =0.514) for a correlation between parents and adoptive offspring in polygenic scores, we provide support for the validity of at least some adoption studies in establishing causal inference.

That is to say, the idea that adopted kids are placed with adopting parents of apparently similar ability is not supported by these findings. (Critics said that the finding that identical twins reared apart correlated in intelligence was not due to their genetics, but to them having been placed in similarly bright families).

On the very important question as to whether adoption has a long-term impact on the ability of adopted children, the answer appears to be that no such effect can be found.

By examining parent-offspring resemblance in a sample of offspring that are among the oldest of any adoption study of IQ to date, we have effectively tested for the presence of parenting effects that would have persisted for more than a decade after the conclusion of the typical rearing period. No such persistence is found to occur in our unique sample.

This is pretty clear. It is unlikely that adoption has long-term effects on adopted children’s intelligence. This should give pause to all those proposing interventions less intensive than the full time parenting provided by adoption and expecting great results.

Source: James Thompson – The Unz Review