Picture a person with an IQ of 100. What's your mental image of the person, bright or not? Now realize 50% of the people are dumber than that person.

Then again, I think IQ doesn't tell us much if anything, and first we need to define what's intelligence. I like to think of it as the capacity and potential, think the CPU and its capabilities. Wisdom is then what you do with that capability, how you apply it. Let's say we did have an easy and precise measurement of the pure intelligence capacity of a person, and put all people on the planet on a gaussian curve. Filter out fringe anomalies and mental conditions from both ends of the spectrum and compare our smartest to our most stupid. My claim is that the difference would be surprisingly low. What we mainly mean when we talk about intelligence is wisdom, and that can be acquired by almost anyone, obviously with some more effort for some. I think the people we mostly think of as intelligent just have a couple other attributes that drive them, curiosity and an open mind. Most people we think of as stupid are quite often not lacking in intelligence, they're lacking in the other attributes.