So... today Charlie Stross asked the question "Why are there so many stupid people?" and I came up with this:
Once upon a time, I took a course in Developmental Psychology. (I'm dredging up memories from long ago. So, I will qualify the following with the fact that it isn't the most up to date information. Also? Memory isn't a 100% reliable thing.)
I seem to recall that a definitive definition of intelligence is virtually non-existent. There are varying types of intelligence and only a small portion of them are measurable. So, we're starting off on shifty ground here.
Why are there so many stupid people?
That said, intelligence is a bell curve. There will always be outliers, and as someone else said--those at the upper end of the bell curve are more likely to have a different perception of the rest of the bell curve than those positioned in other parts of the bell curve. The wide spectrum of intelligence has (probably genetic) advantages or it is likely it wouldn't exist. Thus, there will always be 'stupid' people. Intelligence is not the sole indicator of other valuable qualities such as specific talents (art, literature, physical abilities, emotional development, empathy) which are also valuable to humanity at large--basically, intelligence isn't necessarily the best measure of what moves humanity forward as a whole. I'm thinking of the long ago television program called "Connections." Necessity is the mother of invention, after all. What if the components required to advance weren't entirely intelligence based but a combination of other talents? It is next to impossible to develop high technology in the middle of a war zone. Smoking piles of rubble don't exactly provide an abundance of resources for advancement. Creativity (and not just the artistic kind) doesn't thrive under extremely adverse conditions. Example: there's a reason Steve Jobs evolved in a prosperous, peaceful area of California and not in war-torn Somalia--and it isn't because Somalians are less intelligent than Californians.
So, *if* there is a shift toward higher intelligence in humanity as a whole (and I'm not entirely convinced there is,) I would say that the reasons why aren't merely due the smart people reproducing or communicating with one another.
 a certain amount of adversity is required. it's a balance, however. too much is bad as is too little.