Racism, what do you think are the causes?

Racism/ discrimination is generally not accepted to some degree with the notable exception of anti-Semitism. Racism is obvious, but most people pretend they cannot hear or see what is right in front of them. The 'United Against Racism' campaign may have a marginal impact. Possible causes of racism are fear of difference or unknown or people projecting what they've been taught/ indoctrinated to believe or one experience onto an entire group of people. On the other hand racism is used as a defence against anything, to often successfully. A recent again noticeable example if the foolish Wimp who has done this before most notably with his novel a few years back. What do you think are the causes of racism in general? What do you think is behind the discrimination against Irish in Britain?