How do you define stoopid?

My friends and I had a discussion recently about being stupid and how you can even measure such a thing. The topic started when we were arguing about Sarah Palin and whether or not she’s stupid, ignorant, average, etc.

When asked “what percentage of the US population was stupid”; we all came up with different answers: 7% – 20% – 73%. I came up with 7% because I think stupidity is literally a mental disability and not something one can choose not to be. Think Forest Gump.

How do you classify someone as being stupid? Is it literally their mental capacity to retain information? Is it just being ignorant? Is a “redneck” necessarily stupid or just stubborn / ignorant?