Knowing what we know about human irrationality and xenophobia, affirmative action becomes necessary. Institutions have inertia with respect to their gender and race composition. Suppose for instance that in the past there was aggressive sexism which held women out of science. Suppose furthermore that women and men are perfectly with respect to all relevant psychological traits, such that in a fair world there would be an exactly 50/50 gender split. It's irrational to think that once this overt sexism disappeared, the hiring rate would at once spring to it's natural rate of 50/50%.

Instead there would be all sorts of inertial effects. People would associate science with men more than women. There would be a lack of scientific role models for women. Women would tend not to end up going into science, and when they did they would face further problems. The people hiring them would have an unconscious model of how a good scientist looks, behaves and communicates. This model would be gender biased. Existing scientists would simply be used to a male environment and a male style of communication, women would make them uncomfortable. Humans like those who are similar to them, and the existing, senior male scientists, would be better able to relate to, and inclined to mentor, male students.

Even in the (unrealistic) Absence of any conscious sexism, or any actual differences between the two groups, it would take god knows how long for the pre existing biases to equalize. This is grossly unfair to individual women born during the equalizing process, and bad on a societal level as well. Deliberate social engineering is justified. To speed the process up you introduce a slight bias in hiring procedures - where there is more than one qualified candidate, you pick the woman. There may even be benefits just from having people with less simliar thoughts doing science. Our ability to choose future winners is poor, and already full of biases. One more doesn't hurt much, but it does do a lot of good over time. Obviously too much would be a bad thing. Candidates must still be qualified, but affirmative action isn't evil.