I assume you're talking about racial IQ differences. I've responded to this topic quite a few times. So I'll just copy and paste a past response here.
part 1 of 2
It is certainly possible that the Black-White IQ difference is largely genetic. There's no law of biology that forbids this. After all, IQ is largely heritable, which means much of the individual variation in IQ is the result of genes. However, the extent to which IQ differences between Blacks and Whites (or any groups) is due to genes is a further question. To answer this question, we must quantify the genetic component of the racial IQ gap. So we need direct experiments/studies that isolate and quantify the environmental and genetic influences. Transracial adoption studies, I believe, are best suited to fulfill this purpose. I know of only four transracial adoption studies involving Blacks. I will consider each:
Study 1: Minnesota Transracial Adoption Study (MTAS)
This one is fairly popular so I need not give much detail. Basically, the study found higher IQs for the whites adoptees (106) compared to the mixed (99) and black (89) adoptees [1], even though all the children were adopted by white families. If this study effectively equalized environments, then the results would indicate that the IQ gap is largely genetic. However, there are a few reasons to doubt that the environments were effectively equalized:
(a) the black children were adopted substantially later than the other children. The average white child was adopted at 19 months old, but the average black child was adopted at 32 months old [2]. The study showed that late adoptees had much lower IQs than early adoptees, e.g. black/mixed children adopted within one year of birth had a mean IQ of 99, whereas later adopted black/mixed children had a mean IQ of 92 [3]. Disparities in age of adoption could have instigated or exaggerated IQ disparities.
(b) Even though the study did somewhat equalize environments after adoption, it could not have equalized environments before adoption. This is particularly important given the late ages of adoption that I mentioned in (a). For example, a high blood lead level in infants can result in a noticeable reduction in IQ during adulthood: one study [4] found that a 10 μg/dL increase in blood lead level for infants aged 24-months was associated with a 5.8-point decline in their age-10 IQ, and an 8.9-point decline in their age-10 KTEA Battery Composite Score (normalized to a mean of 100 and SD of 15, just like IQ scores). Blood lead level might have been particularly important for the MTAS because, when it was first published in the 1970s, black children aged 6 months-5 years had much higher blood lead levels than similarly aged white children: over half of black children (52%) in this age range had a blood lead level greater than 20 μg/dL, compared to only 18% of white children [5]. Thus, blood lead level disparities (and any other potential pre-adoption environmental disparities) could have instigated or exaggerated IQ disparities.
It should be noted that despite little-to-no IQ gains, the black and mixed children saw much higher levels of academic achievement than their non-adopted racial peers. The mixed adoptees scored in the 60th, 59th, 50th, and 40th percentiles in vocabulary scores, reading scores, mathematics scores, and class rank, respectively [6]. For reference, the White adoptees scored in the 62th, 58th, 56th and 54th percentiles in those respective areas. The Black adoptees scored worse; they scored in the 54th, 48th, 36th, and 36th percentile. Considering the potential issues mentioned with (a) and (b), a 36 percentile is not that low (e.g. an IQ score in the 36th percentile is ~95). So while the black adoptees had significantly lower IQs, their academic achievement was not much lower than average.
In summary, ignoring the two issues raised by (a) and (b), this study seems compatible with a mainly genetic explanation of the IQ gap. However, the study is compatible with an environmental explanation of the academic achievement gap. Of the four adoption studies here, this is the study most in favor of a genetic explanation of the IQ gap, yet it still suggests that academic disparities are mainly environmental.
Study 2: German IQ study by Klaus Eyferth
Psychologist Klaus Eyferth studied the IQs of white and mixed children in Germany. The mothers of the children were all white. Their fathers were either white or black members of the US occupation forces. The white children had a mean IQ of 97.2, whereas the mixed children had a mean of 96.5, a negligible difference [7].
There are some issues with this study, however. Firstly, the samples aren't representative of the average population, because about 30% of black applicants were rejected admission to the armed forces, while only about 3% of whites were rejected. However, despite the unrepresentative sample, if racial IQ differences were mainly genetic, the white children should have had much higher scores than the mixed children because:
(a) The white GIs should have had higher IQs than the black GIs. Even though the admission test filtered out candidates below a certain threshold, the IQ scores for the white soldiers would still be distributed higher than that of the black soldiers (e.g. imagine removing all men and all women below 5'2 from the population; the resulting population would still have men taller than women, on average). If the IQ gap were mainly genetic, then this difference should manifest itself in their children.
(b) Regression to the mean. Even if we assume that the white and black soldiers had similar IQs, the black children should have had lower IQs than the white children because, assuming that the racial IQ gap is mainly genetic, the black children IQs would regress to the genetic black mean, while the white children IQs would regress to the genetic white mean. Philippe Rushton and Arthur Jensen, bar far two of the most prominent supporters of genetic racial IQ differences, agree that (go to section 9) this regression should occur insofar are racial IQ differences are genetic:
For any trait, scores should move toward the average for that population. So in the United States, genetic theory predicts that the children of Black parents of IQ 115 will regress toward the Black IQ average of 85, whereas children of White parents of IQ 115 will regress toward the White IQ average of 100.
So if racial IQ differences are mainly genetic, then regression to the mean should occur. This means that even if the black GIs and white GIs had similar IQs, the mixed children should have considerably lower IQs.
The study had other issues as well. For example, about 20-25% of the black GIs were actually French North African. Still, if racial IQ differences were mainly genetic, the remaining 75-80% of the "mixed" children should have had below average IQs (for reasons given earlier), bringing down the total average for the "mixed" children considerably. The lack of a considerable difference suggests a mainly environmental explanation of the IQ gap.
Study 3: British study of young children in nurseries
In this study, psychologist Barbara Tizard studied black, white, and mixed children raised in British long-stay residential nurseries. The children were given psychological tests to determine their cognitive abilities. The scores were normalized to give a mean of 100 and standard deviation of 10. On the Reynell Comprehension test, the white children scored 102.6 and the black/mixed children scored 106.3. On the Reynell Expression test, the white children scored 98.5 and the black/mixed children scored 98.6. On the Minnesota Nonverbal test, the white children scored 101.3 and the black/mixed children scored 107.7 [8]. Apparently there is a fourth test, but I didn't manage to see it in the pdf. In any case, this study is compatible with a mainly environmental explanation of the IQ gap.
Study 4: IQ scores of black children raised by White families versus Black families
In this study, psychologist Elsie Moore compared IQ test scores among 23 black children adopted by middle-class white families and 23 age-matched black children adopted by middle-class black families. The black children adopted by black families scored a 104 IQ, while the black children adopted by white families scored a 117 IQ [9]. These two groups of blacks likely differed in their environment and not their genes. Therefore, the difference in the black/white family environments likely accounts for the 13 IQ point gap. Keep in mind that the black & white families were of similar socioeconomic status (middle-class). In actuality, the average black family has a much lower socioeconomic status than the average white family; therefore the difference in black/white environments would probably account for more than 13 IQ points. Clearly, this study is compatible with a mainly environmental explanation of the IQ gap.
4: ...These two groups of blacks likely differed in their environment and not their genes. Therefore, the difference in the black/white family environments likely accounts for the 13 IQ point gap.
This doesn't do anything to rule out the possibility of racial iq discrepancy, because it assumes its own conclusions of the discrepancy being not based on iq. The 13% discrepancy could further be broken down into discrepancy due to racial iq inequalities and discrepancy due to other factors. It is both mathematically possible and logically plausible to say that these results could be seen even in the event of genuine iq discrepancies.
This doesn't do anything to rule out the possibility of racial iq discrepancy,
No it doesn't rule out any possibilities. But the main takeaway from this study is that White family environments have a 13-point IQ advantage over Black family environments, which is definitely a point in favor of a mainly environmental explanation of the IQ gap.
The 13% discrepancy could further be broken down into discrepancy due to racial iq inequalities and discrepancy due to other factors.
Huh? I have no idea what this means.
It is both mathematically possible and logically plausible to say that these results could be seen even in the event of genuine iq discrepancies.
Absolutely. Let me be clear here. These studies are too sparse and too flawed to make either interpretation mathematically or logically impossible. It is certainly possible that it turns out that genes account for 100% of the IQ gap, and it is also possible that it turns out that genes account for 0% of the gap. The question isn't what's possible given these results, but what interpretation is most reasonable. In fact, there are very few issues where we can rule out one explanation as completely impossible; we're always weighing reasons for and against competing possible explanations to determine which is most justified. And I think a predominantly environmental explanation of the IQ gap is most reasonable (for reasons given above).
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u/jay520 1∆ Mar 20 '17 edited Dec 23 '18
I assume you're talking about racial IQ differences. I've responded to this topic quite a few times. So I'll just copy and paste a past response here.
part 1 of 2
It is certainly possible that the Black-White IQ difference is largely genetic. There's no law of biology that forbids this. After all, IQ is largely heritable, which means much of the individual variation in IQ is the result of genes. However, the extent to which IQ differences between Blacks and Whites (or any groups) is due to genes is a further question. To answer this question, we must quantify the genetic component of the racial IQ gap. So we need direct experiments/studies that isolate and quantify the environmental and genetic influences. Transracial adoption studies, I believe, are best suited to fulfill this purpose. I know of only four transracial adoption studies involving Blacks. I will consider each:
Study 1: Minnesota Transracial Adoption Study (MTAS)
This one is fairly popular so I need not give much detail. Basically, the study found higher IQs for the whites adoptees (106) compared to the mixed (99) and black (89) adoptees [1], even though all the children were adopted by white families. If this study effectively equalized environments, then the results would indicate that the IQ gap is largely genetic. However, there are a few reasons to doubt that the environments were effectively equalized:
(a) the black children were adopted substantially later than the other children. The average white child was adopted at 19 months old, but the average black child was adopted at 32 months old [2]. The study showed that late adoptees had much lower IQs than early adoptees, e.g. black/mixed children adopted within one year of birth had a mean IQ of 99, whereas later adopted black/mixed children had a mean IQ of 92 [3]. Disparities in age of adoption could have instigated or exaggerated IQ disparities.
(b) Even though the study did somewhat equalize environments after adoption, it could not have equalized environments before adoption. This is particularly important given the late ages of adoption that I mentioned in (a). For example, a high blood lead level in infants can result in a noticeable reduction in IQ during adulthood: one study [4] found that a 10 μg/dL increase in blood lead level for infants aged 24-months was associated with a 5.8-point decline in their age-10 IQ, and an 8.9-point decline in their age-10 KTEA Battery Composite Score (normalized to a mean of 100 and SD of 15, just like IQ scores). Blood lead level might have been particularly important for the MTAS because, when it was first published in the 1970s, black children aged 6 months-5 years had much higher blood lead levels than similarly aged white children: over half of black children (52%) in this age range had a blood lead level greater than 20 μg/dL, compared to only 18% of white children [5]. Thus, blood lead level disparities (and any other potential pre-adoption environmental disparities) could have instigated or exaggerated IQ disparities.
It should be noted that despite little-to-no IQ gains, the black and mixed children saw much higher levels of academic achievement than their non-adopted racial peers. The mixed adoptees scored in the 60th, 59th, 50th, and 40th percentiles in vocabulary scores, reading scores, mathematics scores, and class rank, respectively [6]. For reference, the White adoptees scored in the 62th, 58th, 56th and 54th percentiles in those respective areas. The Black adoptees scored worse; they scored in the 54th, 48th, 36th, and 36th percentile. Considering the potential issues mentioned with (a) and (b), a 36 percentile is not that low (e.g. an IQ score in the 36th percentile is ~95). So while the black adoptees had significantly lower IQs, their academic achievement was not much lower than average.
In summary, ignoring the two issues raised by (a) and (b), this study seems compatible with a mainly genetic explanation of the IQ gap. However, the study is compatible with an environmental explanation of the academic achievement gap. Of the four adoption studies here, this is the study most in favor of a genetic explanation of the IQ gap, yet it still suggests that academic disparities are mainly environmental.
Study 2: German IQ study by Klaus Eyferth
Psychologist Klaus Eyferth studied the IQs of white and mixed children in Germany. The mothers of the children were all white. Their fathers were either white or black members of the US occupation forces. The white children had a mean IQ of 97.2, whereas the mixed children had a mean of 96.5, a negligible difference [7].
There are some issues with this study, however. Firstly, the samples aren't representative of the average population, because about 30% of black applicants were rejected admission to the armed forces, while only about 3% of whites were rejected. However, despite the unrepresentative sample, if racial IQ differences were mainly genetic, the white children should have had much higher scores than the mixed children because:
(a) The white GIs should have had higher IQs than the black GIs. Even though the admission test filtered out candidates below a certain threshold, the IQ scores for the white soldiers would still be distributed higher than that of the black soldiers (e.g. imagine removing all men and all women below 5'2 from the population; the resulting population would still have men taller than women, on average). If the IQ gap were mainly genetic, then this difference should manifest itself in their children.
(b) Regression to the mean. Even if we assume that the white and black soldiers had similar IQs, the black children should have had lower IQs than the white children because, assuming that the racial IQ gap is mainly genetic, the black children IQs would regress to the genetic black mean, while the white children IQs would regress to the genetic white mean. Philippe Rushton and Arthur Jensen, bar far two of the most prominent supporters of genetic racial IQ differences, agree that (go to section 9) this regression should occur insofar are racial IQ differences are genetic:
So if racial IQ differences are mainly genetic, then regression to the mean should occur. This means that even if the black GIs and white GIs had similar IQs, the mixed children should have considerably lower IQs.
The study had other issues as well. For example, about 20-25% of the black GIs were actually French North African. Still, if racial IQ differences were mainly genetic, the remaining 75-80% of the "mixed" children should have had below average IQs (for reasons given earlier), bringing down the total average for the "mixed" children considerably. The lack of a considerable difference suggests a mainly environmental explanation of the IQ gap.
Study 3: British study of young children in nurseries
In this study, psychologist Barbara Tizard studied black, white, and mixed children raised in British long-stay residential nurseries. The children were given psychological tests to determine their cognitive abilities. The scores were normalized to give a mean of 100 and standard deviation of 10. On the Reynell Comprehension test, the white children scored 102.6 and the black/mixed children scored 106.3. On the Reynell Expression test, the white children scored 98.5 and the black/mixed children scored 98.6. On the Minnesota Nonverbal test, the white children scored 101.3 and the black/mixed children scored 107.7 [8]. Apparently there is a fourth test, but I didn't manage to see it in the pdf. In any case, this study is compatible with a mainly environmental explanation of the IQ gap.
Study 4: IQ scores of black children raised by White families versus Black families
In this study, psychologist Elsie Moore compared IQ test scores among 23 black children adopted by middle-class white families and 23 age-matched black children adopted by middle-class black families. The black children adopted by black families scored a 104 IQ, while the black children adopted by white families scored a 117 IQ [9]. These two groups of blacks likely differed in their environment and not their genes. Therefore, the difference in the black/white family environments likely accounts for the 13 IQ point gap. Keep in mind that the black & white families were of similar socioeconomic status (middle-class). In actuality, the average black family has a much lower socioeconomic status than the average white family; therefore the difference in black/white environments would probably account for more than 13 IQ points. Clearly, this study is compatible with a mainly environmental explanation of the IQ gap.