In Idiocracy, the "average" IQ score falls below 100, despite 100 being the definition of average IQ. However, this could very well be the result of the testers not adjusting the scoring system to account for everybody's stupidity, because their intelligence has diminished as well. — Freiberg, who freely admits to ripping it off of the You Fail The IQ Test page.
Dr Lexus attempts to comfort Joe Bauers by telling him of other 'tards' living 'kickass lives' - including one who became a pilot. Given the inverted logic of the tests, this isn't just a throwaway line - people sharing similar traits to Joe (ie, normal intelligence by modern standards, or close to it) might well be out there doing the jobs that are just a little too complicated for the rest of the population. Certainly there's someone out there at least smart enough to fly a jumbo jet during the film, although since it gets shot down we'll never know if they were smart enough to land it. —Samfnz
Maybe part of the reason for the decline in intelligence and capabilities is that the best people were sent off to colonize other planets and were thereby removed from the gene pool.