Reese is only pretending to be very unintelligent. Malcolm In the Middle
Throughout the Series, Reese appears to be a stereotypical over the top idiot. But maybe he's just pretending to be stupid. Why pretend? Malcolm is miserable, everyone picks on him. The other characters run to Malcolm to fix their problems. Malcolm has tons of pressure placed on him By Lois and Hal. Reese has at times shows he may actually be very smart. At one point he got an absolute Zero on a test so he would have do another year of school. Which getting an absolute Zero would mean he knew enough to get literally every answer wrong on purpose. In season four, he reveals that he simply thinks of a TV advertisement when something difficult comes up. Malcolm does this, and starts to act like Reese. Nobody expects anything of Reese, he's also seemingly much happier then Malcolm. Plus he is also a lot more popular then Malcolm. I don't think he is actually all that dumb, he's just faking it so he can have an easier life.