First of all, affirmative action calls for the hiring of qualified people. Employers tend to hire people like themselves and to think of them as the most qualified. It's harder to see and trust the qualifications of someone who is different. Affirmative action pushes employers to try harder, to cast a wider net. The current conception of merit based on test scores, grade point averages or other measurable standards is too narrow.
Many tests are inadequate to predict success.
Numerous studies have found that there is only a slight relationship between test scores and performance or professional achievement. On the other hand, there is a major relationship between race, income level, educational resources and test scores. Over-reliance on test results inhibits employers from considering other factors that indicate competence and predict success, such as prior work experience and specialized training. Regarding college admissions, tests are culturally biased in that they tend to reflect the experiences of middle class students and their access to higher quality education than that available to less advantaged students. And students are frequently admitted on the basis of many preferences that have nothing to with affirmative action, such as personal connections, financial contributions, geographical diversity, athletic skill, or whether an applicant is a veteran.