We all have prejudices. Stereotypes are a necessary part of our psychological make-up and prejudices are an unfortunate side effect of these. However, we are sentient beings who should be able to rise above these prejudices, see them for what they are, and act in a fair and unbiased manner. Right? Weeeell… sooooort of.
Imagine you are interviewing people for a job and you have in front of you a skinhead man with tattoos and piercings in places you didn’t even know were potential sites for such things. You may well hold a stereotype in your head that tells you people who look like that are aggressive anti-establishment types, which isn’t what you really require for the running of the local Conservative council.
Hang on, you tell yourself, I must not be biased and judgemental. I must not pander to my thoughts that skinheads are aggressive. I must not think that. In the short term that may work and this one lucky skinhead may get a fair hearing. What you are doing, however, is reminding yourself of your prejudice. It is like trying not to think of an elephant. It makes you think of one (this is called ironic process theory). Trying not to think of skinheads as aggressive may actually bring to mind memories from TV or film or real life when you did see skinheads being aggressive. And bringing those images to mind strengthens the stereotype.
So fighting against your prejudice may have, perversely, made it worse.
The good news is that repeatedly, consciously countering the negative stereotype does appear to get rid of it eventually. Unfortunately, however, we hold so many stereotypes, there is no way we could even know what they all are, let alone combat them all.