While at first glance the humble janitor of Paddy's Pub, Charlie Day, may seem to be a healthy, normal, male enjoying his mid-thirties with a beer and some friends, even casual followers of It's Always Sunny in Philadelphia are aware of certain subtle, but persistent behavioural oddities indicative of a deeper disturbance.
Many have commented on the peculiarities, but none have gone so far as to trace it's origin back to a developmental disturbance in the earliest stage of his life. However, having conducted such a study, and made some startling findings, I present them now freely to you, my esteemed colleagues...
In season three, episode one, Charlie discovers a traumatic event of his early childhood in which he had, reportedly, "survived an abortion." If these claims are true, it not only presents an unprecendeted case of medical malpractice, but suggests a biological explanation for his limited cognitive capacity. There is a well known theory in evolutionary embryology coined by Ernst Haeckel called Recapitulation Theory, and it states, in the famous words of Dr. Haeckel himself, that "ontogeny recapitulates phylogeny," or in plain terms, that the stages of embryonic development of a fetus resemble the stages of evolution experienced by their remote ancestors.
That is to say that shortly after conception, a fertilized ovum resembles little more than a bacterium, but as time passes it develops into a tadpole-like creature, followed by one more reptilian in nature, subsequently followed by a primitive mammal, before finally developing, neurologically-speaking, into what is classically considered a humanistic form both in terms of body but especially in terms of brain structure. It is not until the very end of development that the prefrontal neocortex, considered the defining trait of humanity, finally blossoms out from the reptilian brain stem, and it is this crucial final phase of development that was likely compromised by Day's attempted abortion.
And herein lies our hypothesis, that as a result of this interrupted final stage of encephalogical development, Day was born with a brain resembling that of a primitive mammal, similar to a cat or rodent, which explains his preoccupation with small mammals, and rudimentary language processing ability. It is possible that, sensing an innate difference from his fellow man, he subconsciously chose a line of work and habitat that would enable him to spend his time hunting rats, eating cheese, and scurrying about the sewers with his true peers.
Further support to this claim may be found in episode eight of season five in which Day reveals, during a psychiatry session, that he has suppressed urges to become a cat, by gluing fur all over his body and running along with other cats in the night.
It is possible that Day suffers from what may be the first documented case of suppressed trans-species behaviour, almost as if he has the brain of some other mammal trapped in a man's body, and that this identification with an alternate form explains why he is seeking cosmetic alterations to his body. However, his condition, if accurately identified, would make him the first - what may be termed - Transpeciel human, and he would likely suffer great social persecution if such an operation were to take place. Particularly from his closest friends.
And thus, whether ultimately a function of societal pressures to conform or mental inability to identify his predicament, Day is resigned to continue his charade of normalcy, seeking comfort in life's simple pleasures, namely his diet of cheese, cat food, and self-purported favorite food "milk steak."