Well, Down’s Syndrome, for example, clearly affects IQ. There’s a big genetic IQ difference that is only really relevant to individuals. There aren’t Down’s-magnitude intelligence variations between races.
In general, there is wide variation in intelligence among people within any particular ethnic group. Showing these to be genetic doesn’t seem to be too hard either, since you can find individuals of the same ethnic group having been raised in similar environments. On the other hand, the difference in average IQ between races is quite small, compared to the individual within-race differences. To show how much of this was genetic would require controlling for environment, which one can even now expect to be notably different between races.
To put it simply, it’s easy to demonstrate genetic influence, when the effects are of a magnitude such that one can just rule out environment as being the critical factor. Which is not the case for racial differences.
Obviously, you can expect there to exist on average a non-zero genetic component between races, since their genetic material has had time to drift apart. But that’s neither here nor there when you want to know how much.
The point is that even if the heritable component of (say) intelligence among white people formed a bell curve, and the heritable component of intelligence among black people formed a bell curve, a priori you’d expect the two curves to be pretty much the same.
Well, Down’s Syndrome, for example, clearly affects IQ. There’s a big genetic IQ difference that is only really relevant to individuals. There aren’t Down’s-magnitude intelligence variations between races.
In general, there is wide variation in intelligence among people within any particular ethnic group. Showing these to be genetic doesn’t seem to be too hard either, since you can find individuals of the same ethnic group having been raised in similar environments. On the other hand, the difference in average IQ between races is quite small, compared to the individual within-race differences. To show how much of this was genetic would require controlling for environment, which one can even now expect to be notably different between races.
To put it simply, it’s easy to demonstrate genetic influence, when the effects are of a magnitude such that one can just rule out environment as being the critical factor. Which is not the case for racial differences.
Obviously, you can expect there to exist on average a non-zero genetic component between races, since their genetic material has had time to drift apart. But that’s neither here nor there when you want to know how much.
I keep hearing people say that and always wanted to ask which statistics are being compared.
Not about intelligence specifically, but I believe this was the first (well-known) paper making the claim: http://www.philbio.org/wp-content/uploads/2010/11/Lewontin-The-Apportionment-of-Human-Diversity.pdf
The point is that even if the heritable component of (say) intelligence among white people formed a bell curve, and the heritable component of intelligence among black people formed a bell curve, a priori you’d expect the two curves to be pretty much the same.
(Lewontin’s other conclusion, that “race” is “biologically meaningless”, is separate and doesn’t work because what small racial differences there are are statistically clustered: http://onlinelibrary.wiley.com/doi/10.1002/bies.10315/abstract;jsessionid=831B49767DB713DADCD9A1199D7ADC49.d02t02)