Most Helpful Guy
It's actually an evolutionary trait. If our ancestors were threatened by another being or situation and survived, they continued to survive by avoiding similar beings and situations. Those who repeated the same mistakes were eventually killed, removing their particular idiot gene from the potential gene pool.
Extra dumb people have little to no critical thinking skills and rely heavily on stereotypes the way early humans did. Unfortunately, we protect these idiots now, so there's plenty of stoopid going right back into the gene pool. The best we can do is choose our partners very carefully and hope inbreeding eventually wipes out those that are of no use to our species.