[It's Always Sunny in Philadelphia] Charlie starts off as a normal person, then Mac and Dennis give him brain damage.

Season 1 of IASIP begins with Charlie portrayed as a person of average or probably a bit less than average intellect. There are pretty much no signs that he is either illiterate or the complete moron he ends up being. In season 2 this carries over, at least until episode 5. In episode 5, "Hundred Dollar Baby", Dennis and Mac attempt to train Charlie up to fight in an underground fighting competition. To do this, they get him drunk and beat the living shit out of him by breaking wood on his back, hitting him with chairs and smashing him in the head with a metal bin. By the end of the episode, he is irritable, aggressive and having mood swings. He also has very red and bruised periorbital areas (around the eyes). Granted, this is explained within the episode by the steroids he also took. However, these are also signs of raised intracranial pressure, discussed below.

In the **opening scene** of the very next episode, "The Gang gives back", Charlie for the first time is seen having trouble reading a letter regarding community service, particularly struggling with reading "interstate sanitation" and "6 months", as well as not knowing what AA (alcoholics anonymous) is. His illiteracy is made pretty obvious.

My theory is that when bashing him around the head, Mac and Dennis inadvertently caused a small intracranial haemorrhage. This would cause raised intracranial pressure, and may have lead to some brain cell death (possibly in an area necessary for literacy and some other higher brain functioning). The red bruising around his eyes could be what is known as "[Raccoon's sign](https://www.google.com.au/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=O8H7nkIVH2WcLM&tbnid=XgNCrQ1QZGR0mM:&ved=0CAUQjRw&url=http%3A%2F%2Fdoctorsgates.blogspot.com%2F2011%2F02%2Fraccoon-eyes-sign-for-basal-skull.html&ei=1vAOVKGIAcf98AWbzYLYAg&bvm=bv.74649129,d.dGc&psig=AFQjCNFJ3Nae7WWxmXPiEAewYvgtdQAobQ&ust=1410351689794881)" (Warning, a bit gore-y), which is a sign of a base of skull fracture and something similar Charlie seems to be experiencing by the end of the [episode](https://www.google.com.au/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&docid=IxxW6BFCpqvDHM&tbnid=0ph7Zf8SHl5JsM:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.blingcheese.com%2Fimage%2Fcode%2F199%2Fsteroids.htm&ei=L_EOVMLDC4H88AXxhIHIAw&bvm=bv.74649129,d.dGc&psig=AFQjCNGbHhrEBAjx8ltvZBVorbPyzsKspw&ust=1410351776279314). The raised intracranial pressure exacerbated his impaired neurological functioning (hence the mood swings, aggression, psychosis etc) and killed a few cells. Once that had settled down, only the brain cell death was left, hence why he functions normally the next episode but illiterate and slowly getting crazier and crazier.

Anyway let me know what you reckon and if there's anything I missed!

Edit: A lot of people are bringing up the huffing paint and abortion survivor thing, the only things I feel against these, although I think they were the writers' way of writing in his general mental dysfunction, are that he functioned pretty normally before season 2, and then went down here from then. Huffing glue is never mentioned before then, then you see him start to do it. They then say that he did it in high school, but that could have been once or twice and then they bust his balls by saying he "did it all the time". So his abuse of glue could have started after his head injury, as often happens in real life.