a priori there is not reason to expect populations that have historically been geographically separate to have the same distribution of IQ.
I am not sure how much is known about what and how exactly causes IQ (in healthy individuals). With other traits there seem to be some natural limits. For example people don’t have exactly the same average height, but there are some reasonable limits; I don’t know about a population where the average height would be 2 meters, or 1 meter. Various groups of people have huge geographical differences for millenia, there is a variability among individuals of the same population, the nutrition and health contributes to height… and yet, despite all of this, the averages of various populations are within some height interval.
Given this, it does not seem so unlikely to me that there may be similar interval for intelligence. Maybe the interval is more narrow; maybe it is between 95 and 105. Below this interval, higher intelligence is an evolutionary advantage within the population. Above this interval… as I said, I don’t know what causes IQ, but I can imagine that there may be some cost in metabolism, or something similar.
So my prior expectations would be that the distributions are not exactly the same, but there may be some reasonable interval for them. Now the only question is how big that interval is. A difference of 20 points or more would be obvious, a difference of 3 points or less would be hard to notice.
Note: While explaining the genetical differences in IQ, we should also explain why the differences aren’t even greater than they seem. Because the explanation of “different IQ is caused by geographical separation” does not explain e.g. why we don’t have any population with an average IQ of 150. (And if there are reasons why average IQ cannot be greater than 150, maybe there are also reasons why it cannot be greater than 105.)
Various groups of people have huge geographical differences for millenia, there is a variability among individuals of the same population, the nutrition and health contributes to height… and yet, despite all of this, the averages of various populations are within some height interval.
To add to this, while there have been, and continue to be, pretty significant height differences between populations, those differences tend to decrease sharply when the nutrition levels and lifestyles of those populations become more similar. For instance, while a hundred years ago, Americans tended to tower over the Japanese (American soldier second from the left, Japanese soldier far right,) with an average height difference of about six or seven inches, the average height difference today is only about two and a half inches. Even that remaining difference is likely to be at least partly due to a difference in nutrition and activity (the average Japanese person still has a significantly different diet than the average American, and schools demand much higher levels of physical activity of their students, although many adults become highly sedentary after high school graduation.) Unfortunately, I haven’t been able to find any source for the average height of Japanese Americans today, which would help narrow down how much of the remaining gap is likely to be due to lifestyle.
Unfortunately, I haven’t been able to find any source for the average height of Japanese Americans today, which would help narrow down how much of the remaining gap is likely to be due to lifestyle.
If you can get past the paywall, this might give you what you’re looking for. Looks like a pretty small sample, though, and adult height might not correlate that well with childhood height depending on what age we’re looking at. Also a pretty old study; it wouldn’t surprise me if nutrition had changed quite a bit since 1995.
I am not sure how much is known about what and how exactly causes IQ (in healthy individuals). With other traits there seem to be some natural limits. For example people don’t have exactly the same average height, but there are some reasonable limits; I don’t know about a population where the average height would be 2 meters, or 1 meter. Various groups of people have huge geographical differences for millenia, there is a variability among individuals of the same population, the nutrition and health contributes to height… and yet, despite all of this, the averages of various populations are within some height interval.
Given this, it does not seem so unlikely to me that there may be similar interval for intelligence. Maybe the interval is more narrow; maybe it is between 95 and 105. Below this interval, higher intelligence is an evolutionary advantage within the population. Above this interval… as I said, I don’t know what causes IQ, but I can imagine that there may be some cost in metabolism, or something similar.
So my prior expectations would be that the distributions are not exactly the same, but there may be some reasonable interval for them. Now the only question is how big that interval is. A difference of 20 points or more would be obvious, a difference of 3 points or less would be hard to notice.
Note: While explaining the genetical differences in IQ, we should also explain why the differences aren’t even greater than they seem. Because the explanation of “different IQ is caused by geographical separation” does not explain e.g. why we don’t have any population with an average IQ of 150. (And if there are reasons why average IQ cannot be greater than 150, maybe there are also reasons why it cannot be greater than 105.)
To add to this, while there have been, and continue to be, pretty significant height differences between populations, those differences tend to decrease sharply when the nutrition levels and lifestyles of those populations become more similar. For instance, while a hundred years ago, Americans tended to tower over the Japanese (American soldier second from the left, Japanese soldier far right,) with an average height difference of about six or seven inches, the average height difference today is only about two and a half inches. Even that remaining difference is likely to be at least partly due to a difference in nutrition and activity (the average Japanese person still has a significantly different diet than the average American, and schools demand much higher levels of physical activity of their students, although many adults become highly sedentary after high school graduation.) Unfortunately, I haven’t been able to find any source for the average height of Japanese Americans today, which would help narrow down how much of the remaining gap is likely to be due to lifestyle.
If you can get past the paywall, this might give you what you’re looking for. Looks like a pretty small sample, though, and adult height might not correlate that well with childhood height depending on what age we’re looking at. Also a pretty old study; it wouldn’t surprise me if nutrition had changed quite a bit since 1995.
Something, but not much.