In his excellent book The Mismeasure of Man Stephen Jay Gould provides a great historical critique of intelligence testing. His focus is not so much on the tests themselves but on the fallacious conclusions which early 20th century scientists all too eagerly drew from them. Again and again he shows that supposedly objective proofs for the inequality of races were based on nothing more than prejudiced interpretations of statistical data. Some scientists resorted to direct falsification, but it is striking how many acted in good faith without any particular political agendas on their minds, yet still ended up concocting results which were clearly influenced by racist prejudice.
The generally accepted preconceptions of the times predisposed people to interpret the data in a certain way. Since most Africans lived in huts and utilized only primitive technology it was clearly unthinkable that they could be endowed with mental capacities comparable to the white man. An immediate consequence was that any given study of intelligence had to be defective if it didn’t show a clear difference between white and black. The properly scientific thing to do was then to revise the testing procedure or the results until the differences – objectively true as they were, by virtue of common sense – could be discerned.