Just to add, some historians/socialists also hypothesize that those who survived the harsh conditions of slavery not only had a strong physique, but
likely had a disposition most commonly associated with ppl with a low IQ. Psychological descriptions of slaves often mentioned in history were: 'happy go lucky', childish, didnt think about the future (since you really had none, now that you were a lifetime slave. hmmm.."didnt think about the future" correlates strongly with low intelligence.). Anyone with a normal-to-high IQ would become insane, suicidal, revolt, etc, if they were forced to become a slave for the rest of their lives. So you're killed off since you're of no use as a slave.
Normal U.S. Immigration Policy is Good?- putting aside the double-standard mexican immigration issue for a moment

From what I understand, you need to prove you will contribute as a productive citizen in order to immigrate to the U.S. .
African immigration to the United States - Wikipedia, the free encyclopedia
I hear most African immigrants are soft spoken, introverted, and intelligent. I also hear that they don't get along very well with Black Americans (natives). Cultural and psychological differences? Normal-to-high IQ vs Low IQ mindset/values? Both are 'black' biologically. I hear the same with West Indies immigration policies. You need to show you can contribute (be entrepreneurial) and have $$ ($50k?) to come into the country. Once again, very resourceful. very different form the natives (black american culture). yet both are black genetically.
It's Nurture, folks. Or selective immigration.

I do think the slave owners practiced eugenicism for low levels of intelligence. They also forcefully 'bred' strong males with strong females. It's a dark part of history for a reason but it does prove that human evolution actually happens quite quickly.
We also have to remember that there were a handful of dogbreeds only until they began breeding them selectively and now there are hundreds or more.
The Russians domesticated foxes completely in 50 years, which in turn changed their physical appearance into something more dog like.
Horses were much smaller in the wild etc.
There's this new book I haven't read, called the [ame="http://www.amazon.com/The-000-Year-Explosion-Civilization/dp/0465020429"]10.000 year explosion[/ame], which claims that the major driving force in human evolution was agriculture, since it exponentially increased the number of people, which means more mutations. Africa never had agriculture on the same scale, which meant they could be closer to their starting point.
That doesn't mean the average agricultural human is smarter, we know for example that the Cro-Magnon people of early Europe had significantly larger brains than any living human population.
This is actually a common finding, as domestication goes up, intelligence and aggression goes down. A
wolf is much smarter than a dog.
Domestication usually favors some other traits though such as future time orientation, needed to foresee crop harvest, storing away for worse times etc, but mostly it has to do with sociability and empathy. If we are to believe that morality is only self serving, it means you learn to understand others to better be able to co-operate when you are in close contact.
I actually don't believe the claim that the average African has 70IQ, since it is sort of the same wolf vs dog scenario. If observing only dog and wolf, you'd think the dog was smarter.
I do think Africans lack the same evolutionary traits of empathy and co-operation. I see evidence of this in the ease of which Africans and some African-Americans inflict harm on others. People have no idea the barbaric and cruel violence Africans engage in. This is the 'child like' mindset you spoke off. In one second the township African is laughing and playing around, the next he is setting fire to a suspected thief. This ultra violence of Africans is not reported because it is so disgustingly barbaric it would stop private aid, but it happens very frequently.
This lack of empathy would be normal in hunter gatherer societies where it really wasn't as needed outside the tribe.
As for your other point about 'nurture', that is again what we humans have a unique ability to affect nature by our nurture. We can encourage some behavior, some mating and living strategies which favor intelligence, sociability and such or we can let nature rule and only favor the strong and aggressive. Again, that is were agriculture and trade really changed things.