To elaborate on this, if I remember my history correctly, the majority of slaves were not coaxed or abducted by the slave traders but were sold by their tribal leaders. Those of lesser intellect, higher physical skills, and (it was presumed) those who were malcontents were most likely to be chosen.
Once they reached the "land of the free", slaves were selectively bred to create stronger field workers. In addition (as Chris Rock points out in a humerous yet sad routine) in some areas slaves were routinely lynched if they were found to be able to read and write.
These are facts that not many (including myself) feel free to discuss in a group setting both due to PC repercussions and to keep the information from being selectively used by a hateful racist.
Another non sequitor point to consider is that genetics only determines 50% of an individuals attributes, the other 50% is environmental.