The impact of low birth weight on IQ seems to be about 5 points (see Table 2). Given international adoption is generally* among the neediest, it seems likely that maternal malnutrition/low birth weight makes up a material part of the difference here.
*I haven't seen numbers on this, only the general claim, which is plausible but unquantified.
Bryan, There are many factors that you did not (or could not) include in your analysis, but one that would serve to impact in a negative way the capacity of children adopted internationally is the shock of finding themselves thrust into a foreign language. It appears that children do learn a different language more easily than adults, but the sudden disruption for children over 1-2 years of age would be a shock.
Another factor that is hard to evaluate is the change in nutrition and its effect on children. You did talk about height, weight, and head size differences, but how much effect might improved nutrition have on IQ?
I retired early and moved to Ecuador 13 years ago. It is hard to evaluate IQ in the people I associate with (they don't seem stupid), but one of the chief differences I have noted here is that you find very few bookworms, and nearly all the books available to read are translations from English. So is the general lack of interest in reading a consequence of differences in IQ, or simply that children here don't have the opportunities to read that are widely available in the US?
(1) It seems that at age 18 someone adopted from abroad before age 2 (but not from Korea) has an IQ of about 89, instead of the 84 we'd expect from their home countries' average of the 99 we'd expect if they were average Swedes. That is indeed a big effect. But, you say, from lots of other studies we know that
(2) if the Swedish couple had adopted a Swedish-born orphan with parental IQ of 84, that orphan would grow up to have an IQ of 84, not 89, and
(3) if the Swedish couple had adopted a Swedish-born orphan with parental IQ of 88, that orphan would grow up to have an IQ of 88, not 89.
That's quite interesting. One explanation might be that abroad the average person grows up with very poor nutrition, so their average IQ is 84 where with good nutrition it would have been 89. When a foreigner is brought to Sweden as a toddler, his nutrition improves so he ends up with an IQ of 89. Thus, we might back out a "natural IQ" for his country of 89, not 84, where by "natural" I mean the IQ people would reach if they had optimal nutrition like western countries do. But the Swedish orphan with parental IQ of 84 already had good nutrition before being adopted, so he doesn't get that "good nutrition" bump.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4979774/
The impact of low birth weight on IQ seems to be about 5 points (see Table 2). Given international adoption is generally* among the neediest, it seems likely that maternal malnutrition/low birth weight makes up a material part of the difference here.
*I haven't seen numbers on this, only the general claim, which is plausible but unquantified.
Bryan, There are many factors that you did not (or could not) include in your analysis, but one that would serve to impact in a negative way the capacity of children adopted internationally is the shock of finding themselves thrust into a foreign language. It appears that children do learn a different language more easily than adults, but the sudden disruption for children over 1-2 years of age would be a shock.
Another factor that is hard to evaluate is the change in nutrition and its effect on children. You did talk about height, weight, and head size differences, but how much effect might improved nutrition have on IQ?
I retired early and moved to Ecuador 13 years ago. It is hard to evaluate IQ in the people I associate with (they don't seem stupid), but one of the chief differences I have noted here is that you find very few bookworms, and nearly all the books available to read are translations from English. So is the general lack of interest in reading a consequence of differences in IQ, or simply that children here don't have the opportunities to read that are widely available in the US?
I'm trying to figure out a way to summarize.
(1) It seems that at age 18 someone adopted from abroad before age 2 (but not from Korea) has an IQ of about 89, instead of the 84 we'd expect from their home countries' average of the 99 we'd expect if they were average Swedes. That is indeed a big effect. But, you say, from lots of other studies we know that
(2) if the Swedish couple had adopted a Swedish-born orphan with parental IQ of 84, that orphan would grow up to have an IQ of 84, not 89, and
(3) if the Swedish couple had adopted a Swedish-born orphan with parental IQ of 88, that orphan would grow up to have an IQ of 88, not 89.
That's quite interesting. One explanation might be that abroad the average person grows up with very poor nutrition, so their average IQ is 84 where with good nutrition it would have been 89. When a foreigner is brought to Sweden as a toddler, his nutrition improves so he ends up with an IQ of 89. Thus, we might back out a "natural IQ" for his country of 89, not 84, where by "natural" I mean the IQ people would reach if they had optimal nutrition like western countries do. But the Swedish orphan with parental IQ of 84 already had good nutrition before being adopted, so he doesn't get that "good nutrition" bump.
Have I got it right?
Yes. Anything surprising?