Turkheimer (2003) found that for children of low socioeconomic status heritability of IQ falls almost to zero.
We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples...It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence.
I’ll concede that IQ has a substantial genetic component, as your link says:
Estimates in the academic research of the heritability of IQ have varied from below 0.5 to a high of 0.8 (where 1.0 indicates that monozygotic twins have no variance in IQ and 0 indicates that their IQs are completely uncorrelated).
As you can see in the linked Wikipedia article, in developed countries, adult IQ is more heritable than child IQ. This is not consistent with IQ differences being primarily environmental. If they were, we would expect IQ to become less heritable as environmental factors such as “two decades of very expensive private schooling” accumulate over time.
In poor countries, IQ is on average lower than in developed countries, and idividual IQ differences are less heritable, and they tend to be correlated with observable environmental factors such as malnutrition and infectious diseases.
These observations are consistent with a model where genetic factors largely determine a maximum potential IQ and environmental factors determine how much of this potential people actually reach and how fast they reach it. In developed countries, most people eventually reach a level close to their genetic potential, but people from a more advantaged background reach it faster. In undeveloped countries, environmental factors have a much larger impact and only few people reach their genetic potential. This is probably complicated by epigenetic factors that make the environmental influences carry over, to some degree, to one or two generations.
Human stature follows similar patterns of heritability.
From a purely methodological perspective, I think it is interesting to note that this study does not account for whether the monozygotic twins share the same placenta.
75% percent of monozygotic twins share the same placenta (“monochorionic twins”). , whereas 25% have separate placentas (“dichorionic”). If the twins are monochorionic, there is a real risk of twin-to-twin transfusion syndrome, such that one child steals resources from the other in utero.
If you are studying an outcome that is affected by twin-to-twin transfusion (such as intelligence), including monochorionic twins in the sample will cause substantial bias towards lower correlation. Therefore, if I designed the study, I would have looked only at dichorionic twins. Obviously, this means you would have to throw out 75% of the sample, but I think they should have at least included some figures on how many of the twins were monochorionic, with perhaps a separate analysis using only dichorionic twins to see whether it affected the conclusions.
Twin studies prove you wrong.
“Prove” and “wrong” are such strong words...
From your link:
I’ll concede that IQ has a substantial genetic component, as your link says:
As you can see in the linked Wikipedia article, in developed countries, adult IQ is more heritable than child IQ.
This is not consistent with IQ differences being primarily environmental. If they were, we would expect IQ to become less heritable as environmental factors such as “two decades of very expensive private schooling” accumulate over time.
In poor countries, IQ is on average lower than in developed countries, and idividual IQ differences are less heritable, and they tend to be correlated with observable environmental factors such as malnutrition and infectious diseases.
These observations are consistent with a model where genetic factors largely determine a maximum potential IQ and environmental factors determine how much of this potential people actually reach and how fast they reach it.
In developed countries, most people eventually reach a level close to their genetic potential, but people from a more advantaged background reach it faster. In undeveloped countries, environmental factors have a much larger impact and only few people reach their genetic potential.
This is probably complicated by epigenetic factors that make the environmental influences carry over, to some degree, to one or two generations.
Human stature follows similar patterns of heritability.
From a purely methodological perspective, I think it is interesting to note that this study does not account for whether the monozygotic twins share the same placenta.
75% percent of monozygotic twins share the same placenta (“monochorionic twins”). , whereas 25% have separate placentas (“dichorionic”). If the twins are monochorionic, there is a real risk of twin-to-twin transfusion syndrome, such that one child steals resources from the other in utero.
If you are studying an outcome that is affected by twin-to-twin transfusion (such as intelligence), including monochorionic twins in the sample will cause substantial bias towards lower correlation. Therefore, if I designed the study, I would have looked only at dichorionic twins. Obviously, this means you would have to throw out 75% of the sample, but I think they should have at least included some figures on how many of the twins were monochorionic, with perhaps a separate analysis using only dichorionic twins to see whether it affected the conclusions.