[dragonwriter]

> what about discrimination against stupid people?

It is also very common, but also very frequently is discrimination that goes to the core of the overt purpose of the decision being made. As discrimination is simply having any factor play into a decision, the usual problem is with (1) decisions whose motivation is to cause harm to a group, or (2) decisions in which factors like membership in a group are weighed that are not germane to the overt non-malicious purpose, which cause an (unwarranted by the purpose) adverse impact on the group.