If an alien at this point were presented with these premises, does anyone suppose its natural conclusion would be, "the observed black-white IQ differences must be at the very least partially due to genetic factors, it is not possible that the environment can account for all of it."
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The point is not that a genetic factor is not possible, but that the environmental factors (in our past and current racist world) totally overwhelm any possible genetic factors, even to such a degree that the genetic difference could even go the other way (i.e. blacks could be genetically more intelligent than whites).
I'd personally add this point: Those emphasizing the importance of open-mindedness about the possibility of genetic factors while failing to acknowledge the severity of environmental factors are thus contributing to those very environmental factors. They are self-fulfilling prophets.
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I'm pretty sure if you got in a time machine and went back to the 18th or 19th century, and told people there what blacks, women, and gay people do in the early 21st century, they'd have more trouble believing that than your time machine.