Only someone who hasn't spent much time around people with 2-digit IQ's would believe in "the psychological unity of humankind." The empirical evidence shows that at least in the area of IQ or the General Intelligence Factor (g), marginal differences can have profound practical consequences:
Why g Matters: The Complexity of Everyday Life
http://www.udel.edu/educ/gottfredson/reprints/1997whygmatters.pdf
Nick Bostrom in one of his talks even argues that raising everyone's IQ by 10 points would revolutionize our society for the better, not by making the smartest people a little bit smarter, but by making hundreds of millions of the world's dumbasses substantially smarter so that they would become more educable, develop lower time preferences and make better decisions in life.
It's because what EY meant by the psychological unity of humankind was more along the lines of,
... everyone has a prefrontal cortex, everyone has a cerebellum, everyone has an amygdala, everyone has neurons that run at O(20Hz), everyone plans using abstractions.
We might disagree about the last one, but the first four are pretty much fixed.
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: