There are a few ideas floating around behind concepts like Affirmative Action and desegregation:
- Exposure breeds tolerance: people (as a rule) can be paranoid about people and things they are not familiar with. Forcing members of one group to be exposed to members of another group creates a level of familiarity and comfort that undercuts reactive oppressive attitudes
- Exemplars: getting a few people within an oppressed minority group into positions of power and authority serves as an example to other members of the group that it is possible to rise within society, which encourages higher goals and greater effort across the minority group
- Fiscal ambiguity: in segregated societies, funds for education and public services are invariably allocated disproportionately towards the hegemonic group. Programs like AA make it more difficult to funnel public funds to the benefit of one group over another, because it becomes more difficult to tell which locations are exclusive to one group or the other
- Mentorship and leadership: getting a few people within an oppressed minority group into positions of power and authority means that money, effort, and power tend to flow into the minority community, not away from it. That creates opportunities, raises property values, increases community prestige, and offers other benefits
None of these intuitions plays out quite as expected in the real world — I mean, what does? — but the general principle of reducing alienation and isolation clearly has a slow, cumulative effect on a society. Consider, for instance, that in 1960 there were 19 women and 4 blacks in Congress, and as of today there are 144 women and 65 blacks. While Affirmative Action didn't cause that change directly, it created the attitudes that allowed it.