Human groups have evolved to have average differences in skin colour, hair type, proneness to certain diseases, bone density, fat distribution, musculature, facial structure, oral bacteria, ability to produce vitamin D, response rates to certain medicines, malocclusion occurrence, vocal resonance, dental patterns, rates of hair loss, penis size, rates of myopia, average height, propensity for sweating, breast density, vaginal canal width, hip-to-waist ratio, leg-to-height ratio, muscle fibre type, jaw width, CRANIAL CAPACITY and BRAIN VOLUME...
....but cognitive ability has remained the same across all human groups to the 10th decimal place.
The blank slate, ladies and gentleman
Yeah, I'll steel-man it for you if you like; if you haven't interacted with one of these lefties yourself. Maybe you'd find that interesting.
BUT FIRST: THE TAX! In this video, a couple of girls set up a lemonade stand in their nice suburban neighborhood. They earn $30, and I'm sure their neighbors appreciated how these two girls made the world just a little better.
...a wild black approaches. He announces that he has no money. The girls give him a glass of lemonade for free. He walks away. But then later he returns and robs them - not just robs them, but flips their table over.
I wonder what he was thinking between the time he left, and when he decided to go back and rob them. "sheeeeet, dem yt bitches got money! Dat my money!"
Anyway, back to the "blank slate" discussion.
Human groups have evolved to have average differences
The lefty is going to say, "there's more variation within the group than between any two groups." If you haven't heard that statement before, and you don't know how to counter it, or why it's a fallacy, let me know. For now, I'm just telling you their position.
in skin colour, hair type, proneness to certain diseases
etc. etc. The lefty is going to say:
(1) none of those things is related to intelligence. I realize that is completely ignoring the point you're making. Again, I'm just letting you know what they'll say.
(2) those differences (skin color for example) are on a spectrum. The claim here is that because there are gradations, you cannot make statements about averages. It's like pointing out that height varies millimeter-by-millimeter from one person to the next ...so therefore you cannot say "men tend to be taller than women."
(3) race isn't real. Since you pointed to differences in hair color, you are likely to get a response that points out that there are hair color differences within the group you're calling "white people" and you'll get a claim that, if whites are different from blacks due to (in part) hair color, then all these groups of white people are also different races ...which the lefty thinks renders your argument moot. It's a bit like saying that golden retrievers are different from poodles, so therefore you can't claim that pit bulls are violent.
CRANIAL CAPACITY and BRAIN VOLUME
Just FYI, if you ever say that online, you have to link to a study. If you say it in person, you'd better have the study printed out. Otherwise, bringing that up is an instant and irrecoverable derail, and the lefty screams PHRENOLOGY and drops the mic.
And to a lefty who has done this before (admittedly, that's a rare one), you also need a source other than Rushton.
but cognitive ability has remained the same
The lefty is going to say that cognitive ability is so substantially dependent on environment as to smooth over any population-level differences. If you imagine a concrete floor with some minuscule bumps and depressions making it not perfectly level, then you pile 150 rugs on top of it, the resulting surface is entirely due to the rugs, and it makes no sense to even discuss a tiny bump in the concrete below.
The lefty is going to point out that IQ is effected by things like nutrition. And they're right ...but we can control for literally everything with statistics, so we know very precisely the extent to which nutrition contributes vs. the genetic contribution. But that's what the lefty is going to say.
Oh an also, they'll say that IQ isn't real. "Isn't real" as well as "not scientifically valid" and the old standard, "a social construct" are some of their favorite memes in support of the blank slate.