Intelligence is a difficult attribute to quantify and it's a difficult attribute to understand.
Differences in IQ are probably entirely the function of the environment in which people live. Example, the average IQ gain when a child in poverty is adopted by an upper-middle class family is 12 to 16 points. There is precious little evidence of inherited racial IQ differences that withstand scrutiny.
Here's a review of a recent book examining this issue: http://www.nytimes.com/2009/03/29/books/review/Holt-t.html?_r=1