But somehow it's very controversial that the small intelligence differences between humans might be genetic, as if a species has a fixed IQ number, and the only IQ jumps are strictly between species, but not between individuals.
The flaw is when you start to extrapolate that to things like racial issues. Saying that for example black people are less intelligent due to their genetics would ignore a lot of environmental circumstances that can lead to people not achieving their full potential. There's also the conclusion that if someone is born from a poor and dumb family that therefore their genetics must be worse and so forth.
Framing the argument as 'nature not nuture' would still ignore that if you had two people with identical genes born into a rich family vs a poor family that their theoretical 'genetic potential' is going to be highly limited by their environment. The genetic argument quickly becomes a tool with which to justify hurting others.
That particular assumption would be false because of genetic crossover. Two mentally disabled people could give birth to a genius child if recombination results in an offspring not carrying or expressing whatever causes their disability. The probability of this happening is higher if the pair is less closely related. The inverse can also happen of course, and does.
This is really why sexual reproduction seems to exist. It's also why the racial purity crowd has it precisely wrong. One hypothesis on the Flynn Effect is that it could result from increased outbreeding due to modern levels of travel and migration.
If people are talking about political ideology and wishful thinking, rather than science, then that's another matter. As the article says:
> those on the left have tended to see the environment as the critical factor because it ties in with notions of egalitarianism
I'm not quite sure what you're asking here. I feel that you and I must have drastically different starting assumptions, because your question seems meaningless in my context.
Do you dispute that Homo sapiens has a higher intelligence than H. erectus did? Than other primates?
Or are you alleging that the genetic basis for that intelligence difference shows no variation across human populations?
I feel like I have to stretch what you said to give it a charitable interpretation, but here goes: are you asking whether there's any evidence that the genetic component's variation across human populations contributes to significant phenotypic variation when compared to environmental factors?