Ethnic minorities and women are disadvantaged enough as it is. When considering why members of some ethnic groups tend to do badly in school, and why girls tend to do worse than boys in mathematics, people present all kinds of arguments, including economic, cultural, and sometimes even (very controversially) genetic reasons. A contributing factor that one seldom hears about is the pernicious psychological effect known as stereotype threat.
Stereotype threat describes the unconscious tendency for people to worsen their performance in a task when they are reminded of a negative stereotype that a group to which they belong has. For example, when girls are made to put their gender on the front of a mathematics exam script, then they do worse than when they aren't so asked. This means that merely reminding girls of their gender is enough to make them be momentarily worse at maths, as if subconsciously trying to confirm the stereotype.
This kind of effect has been repeated with other stereotyped-against groups, such as african american and latino children in the United States. Interestingly, the positive side of the effect seems to be very small - white children don't benefit from being reminded that they are white, for instance.
The "reminding" mentioned here can be very subtle. The students don't need to be aware of what is going on - it's a classic subconscious process, like priming. For instance, even getting people to write down what part of town they're from is enough to activate racial stereotype threat.
It should be obvious that there are serious practical implications. One in particular that is worth mentioning is that some examinations require students to write down their gender and/or their ethnicity before starting, which is shown to activate stereotype threat and thereby reduce performance. Apparently this is what happens in the USA with the SAT school-leaving test, although I have had trouble confirming this.
This article describes the phenomenon, and discusses some potential ways of mitigating the effect. Michelle Goffreda originally wrote it as a blog post on the MIT Admissions blog (http://mitadmissions.org/blogs/entry/picture-yourself-as-a-stereotypical-male). With her permission, in this episode I read it out and add my own comments.