The Detroit Area Study asked a sample of 384 blacks and whites whether the word "intelligent" better described blacks or whites. Respondents could also answer "equally descriptive" or "not descriptive of either."
Here are the responses in percentages:
More descriptive of blacks 0.6
Equally descriptive 54.5
More descriptive of whites 33.4
Descriptive of neither 11.4
More descriptive of blacks 19.7
Equally descriptive 68.4
More descriptive of whites 11.8
Descriptive of neither 0.0
It is not surprising that "equally descriptive" is the most popular answer given by both races. Not only is America's culture egalitarian, there is also a tendency to interact with people similar to yourself. Smart people often find themselves among smart people, and slower folks with people like themselves. Until I lived in Newark, I had only met middle-class blacks, and was stunned by the "differentness" of inner-city African Americans. Whites with little exposure to poor blacks might sincerely believe that IQ averages are the same.
Blacks spend more time with whites than the reverse, so they are more familiar with the other race, which might help explain that there are actually a few (11.8%) who admit that whites are more intelligent.
Of course, it's taboo to publicly claim that whites are smarter, but one-third of whites manage to say this to the interviewer.