After the broadcast of "Breezy", I began to notice a trend in Finn's behavior (particularly in Season 5). These trends led me to further researching, and ultimately, I have concluded that Finn's behavior very closely correlates to a personality disorder known as "Borderline Personality Disorder". Here listed is the collected data to back up my claim, with bullet points linking to each occurence in the series' run up to "Furniture & Meat".

Please note that this list has massive spoilers.

Borderline Personality Disorder is a cluster-B personality disorder that often manifests itself during adolescence, and can be brought on by numerous influences, including genetics, brain abnormalities, and adverse childhood experiences. BPD is defined in anyone who displays pervasive impulsivity, and instability of affectsinterpersonal relationships and self image. Sufferers of BPD feel emotions more deeply, and for longer periods than others do. Specifically, Finn seems to suffer from a combination of Petulant and Impulsive BPD.

Finn displays many characteristics of Borderline Personality Disorder, which are collected and expanded upon below. However, this collection of information is in no way meant to speculate or theorize on any unstated aspects of the show's established storyline. In addition, the Adventure Time comics and video games are not included in this list. The information presented here follows the canon in broadcast order, not production order.

Titles of episodes that are bold have major examples of common BPD behavior.

Unusually intense sensitivity to relationships (Emotional Lability/Rejection Sensitivity):

(This section refers only to Finn's romantic relationships)

  • "Ricardio the Heart Guy": Finn's jealous behavior towards Ricardio is noticeably more violent and intense than normal, causing him to stalk Ricardio and scream out in public.
  • "The Real You": Rather than present his own merits at Princess Bubblegum's Science BBQ, Finn chooses to nearly destroy the world with a black hole just to impress PB, showcasing his impulsive and obsessive nature.
  • "Wizard Battle": Despite knowing that he is violating the sanctity of Wizard Battle, Finn joins the battle anyway, all so that he can stop anybody else from kissing Princess Bubblegum under the pretense of stopping the Ice King from "violating PB's lips", even going so far as to furiously scream the victorious Abracadaniel into unconsciousness.
  • "Incendium": After Princess Bubblegum "rejects" his attempt to lie on her lap, Finn falls into an unusually deep depression and surrounds himself with things that remind him of Bubblegum, showcasing his unhealthy infatuation with her, and possibly his high rejection sensitivity. Upon talking to Flame Princess for the first time, he immediately falls in love with her and abandons his depression, suggesting an emergence of emotional lability, which is one of the most common signs of BPD, along with rejection sensitivity.
  • "Hot to the Touch": Finn stalks Flame Princess through the forest, despite the fact that she and Jake explicitly told him to stay away from her. This can also correlate to another symptom of BPD: Impulsive manipulation of people for one's own benefit or enjoyment (see topic below)
  • "Reign of Gunters": After reading "Mind Games" by Jay T. Doggzone, Finn adopts a new stance on his relationships, choosing to keep all the ladies "in a state of confusion, so [he] has options in case Flame Princess doesn't work out". This showcases an emotional detachment to his own relationships, despite the fact that he wishes for them to continue, another example of his impulsivity.
  • "All the Little People": While "playing" with the Little People he received from Magic Man, Finn establishes a relationship between little Lady Rainicorn and his own little self, then forcefully manipulates little Finn into a relationship with little Flame Princess, which he almost immediately breaks by dropping in little Princess Bubblegum between the two, just to see what would happen. This is a very clear example of Finn's view of relationships, showing that if he feels there are no immediate consequences to his actions, he would simply hop from one relationship to another purely for his own curiosity or enjoyment (he refers to the Little People as "toys", despite the fact that they display clear thoughts and emotions). This is an extremely common practice in sufferers of BPD, and seems to be the starting point of Finn's more obvious mental decline.
  • "Puhoy": When Flame Princess does not laugh at Finn's joke, he becomes very depressed and even suggests that their relationship is over, despite the fact that she just didn't get the joke at first; another clear example of emotional lability, also known as splitting or black-and-white thinking.
  • "The Suitor": When Princess Bubblegum reveals one of her suitors named Braco to Finn and Jake, Finn seems automatically disdainful of him, calling him "weird, passive, and baby-lambish... Too unheroic for Peebs.", once again showing his abject jealousy towards anyone who would attempt to form a romantic relationship with Bubblegum (see "Ricardio the Heart Guy" and "Wizard Battle" above).
  • "Too Old": Almost immediately following his breakup with Flame Princess during the events of "Frost & Fire", Finn begins to court Princess Bubblegum once again, showing almost complete ignorance of the surrounding situation involving the Earls of Lemongrab, instead attempting to rekindle the more childish relationship the two briefly had in "Too Young". His attempts are quashed by Bubblegum, who states they are "little boy games", and upon being rejected, Finn instantly suggests patching things up with Flame Princess. This indicates that he doesn't really care who he's with, as long as he's with somebody, and is perfectly willing to hop from one relationship to the next, without giving himself any time to heal mentally and emotionally. This is another classic example of BPD-related behavior.
  • "Earth & Water": When Finn discovers that Flame Princess has taken over the Fire Kingdom, and has already moved on from Finn romantically, he falls to the floor and curls up into a fetal position, displaying another example of emotional lability (he goes from being hopeful to depressed in a matter of seconds).
  • "Love Games": Though Finn claims to want to return to saving princesses instead of dating them, he also admits that he can't move on from Flame Princess and that he "see[s] her face all over Ooo". This once again showcases his rejection sensitivity.
  • "The Red Throne": When Flame Princess is dethroned, she turns to Finn for help. However, he completely ignores the actual importance of the situation in favor of unsuccessfully trying to win back Flame Princess's affections. When Cinnamon Bun admits his love for Flame Princess, Finn mutters darkly, "Did I just get shown up by Cinnamon Bun?", once again showing his disdain for having any romantic rivals.
  • "Breezy": Following the events of "Escape From the Citadel" and "The Tower", Finn seems to have fallen into almost complete dissociation with his emotions, stating that he "feels nothing". He decides to make out with various princesses in order to jumpstart his emotions, but it only serves to depress him further, and nearly causes him to lose his arm permanently. Dissociation, along with intense depression and impulsive relationship hopping, are three of the most common behavioral conditions of BPD sufferers.

Impulsive manipulation of people or events for one's own benefit or enjoyment (Manipulative/Impulsive Behavior)

(Some instances of Finn's impulsive behavior are not listed, such as when he acts impulsively for the benefit of someone else. Listed here are examples of Finn's impulsive behavior when it negatively affects one or more people, including himself.)

  • "The Jiggler": Finn finds a small creature he calls the Jiggler, and suggests taking it home to stick by his bed, so he can wake up "next to a little dancing guy every morning!". He does not even consider the consequences of taking a living creature away from its habitat, and thinks only of his enjoyment of the situation. Only when the Jiggler nearly dies does he think to bring it home.
  • "Wizard": Rather than fulfilling the "Pledge of Ultimate Responsibility" that he took in order to become an Ultimate Wizard, Finn chooses to abandon his post and create his own solution to save the wizard academy from being hit by an asteroid. Even though his plan is ultimately successful, he did agree to take his place in protecting the village, and by leaving his post to enact his own plan, he shows an aspect of his manipulative-impulsive behavior.
  • "Dungeon": Despite Princess Bubblegum and Jake's warnings, Finn still chooses to try and conquer the Dungeon of the Crystal Eye, and recklessly endangers himself purely for his enjoyment.
  • "His Hero": While trying to adhere to his new method of non-violent heroing, Finn feeds the townspeople, and adds way too much stone-skin potion to the gruel, making all of the townspeople grow thick armored skin, even though nobody asked Finn to do such a thing. In addition, he also turns Lumpy Space Princess into a cyborg fighter, even though she merely wanted a "hot swimsuit body". This again showcases his impulsiveness and lack of respect for anyone else's wishes.
  • "It Came from the Nightosphere": Without asking, Finn summons Marceline's Dad into the Land of Ooo in order to have him and Marceline reconnect. He does not even consider the logistics behind their estrangement, and merely acts on a whim for his own impulsive reasoning.
  • "Storytelling": Finn spends nearly the entire episode manipulating the animals of the forest to create a story to cure Jake's sickness. Though he does appear to have quandaries with hurting animals, this could be because the story is not for his benefit directly, but for Jake's benefit.
  • "The Other Tarts": Rather than follow orders and take the safe path for toting the Royal Tarts, Finn decides to take an extremely dangerous path on the assumption that "no thief would expect a tart toter to take [it]". This indicates that he holds much greater priority on his own assumptions than actual facts and does not think his actions through, which is a common attribute of impulsiveness.
  • "The Pods": After accepting a quest to guard magic beans from a dying Gnome Knight, Finn again decides to impulsively use his own methods of dealing with the situation, without thinking of the consequences of his actions.
  • "The Real You": (see related topic above)
  • "Video Makers": When filming a movie for their movie club, Finn and Jake argue over the genre of the film they're making. Finn, however, appears to be much more forceful in his rejection of Jake's desire to make a romantic comedy, calling it dumb and looking visibly annoyed while Jake films his scenes. This is one of the earlier examples of Finn's obstinate rejection of anybody else's desires.
  • "Heat Signature": Rather than sit on an uncomfortable couch, Finn asks to be turned into a vampire so that he can float. This decision is incredibly rash and unnecessary (he could just sit on the floor), showcasing his extreme impulsivity once again by not considering the possible consequences of becoming a vampire.
  • "Wizard Battle": (see related topic above)
  • "The New Frontier": After Jake has his 'Croak Dream', he seems to be content with his own death. Finn, however, tries obsessively to stop the premonition from coming true, even going so far as to shoot at Banana Man with a crossbow. This again shows that he holds little stock in the wishes of others, and chooses to manipulate a situation for his own desires if he feels he is able.
  • "Paper Pete": Instead of simply allowing Jake to study the "Rainicornicopia" in peace, Finn actively tries to get him involved in an adventure, even though Jake isn't interested, ultimately causing him to get partially eaten by the Moldos. This is another clear example of Finn's manipulative behavior.
  • "Another Way": Refusing to receive medical treatment for his broken toe from the Clown Nurses, Finn snaps into a spell of near-insane levels of impulsiveness while searching for magical Cyclops Tears. He refuses to listen to anyone's advice or warnings, constantly screaming "MY WAY!" while he becomes more and more injured, until ultimately he crushes Rainy under rolling melons. Even though he has a moment of clarity, he still retrieves the Cyclops Tears by tearing the Cyclops' head clean off and bringing it home with him.
  • "Hot to the Touch": (see related topic above)
  • "Beyond this Earthly Realm": Despite Jake's warning about the Lamb Relic having some kind of sacred significance, Finn dismisses him by saying he wants it "for his sacred bathroom", showing that Finn's own desires outweigh his respect for sacred artifacts, plus another example of his impulsiveness.
  • "Who Would Win": Infuriated over Jake's lack of enthusiasm for training to fight The Farm, Finn destroys Jake's game "Kompy's Kastle", and he and Jake fight to the point of unconsciousness. This clearly shows once again that Finn has little to no regard for anyone else's desires, and will become violent if need be in order to defend his own stance on the situation. This also shows that he is not above using psychological manipulation to achieve his own goals, which is another common sign of BPD sufferers.
  • "All the Little People": When given a bag of Little People by Magic Man, Finn proceeds to manipulate nearly every aspect of their lives, including who they have relationships with. He shows nearly complete indifference to the Little People's suffering for sixteen weeks while he manipulates their lives alone purely for his own curiosity and enjoyment. He dismisses his actions to Jake by calling them "toys", despite the fact that they clearly exhibit thoughts and emotions. This episode showcases that Finn's manipulative behavior can rise to dangerous levels if he does not have to worry about any consequences of his actions, and seems to be the starting point of his more obvious mental decline.
  • "Vault of Bones": When Finn takes Flame Princess to a dungeon to try and prove that she isn't evil, he spends almost the entire time doing things his way, despite the fact that Flame Princess is clearly bored and not enjoying herself. When he finally allows Flame Princess to do things her way, he is noticeably uncomfortable and skeptical of her methods, despite the fact that they appear to be working well, albeit dangerously. This is another example of Finn's impulsive rejection of anyone else's methods.
  • "Jake Suit": When Finn wears Jake as a suit of "armor", he is completely oblivious to the fact that he is hurting his friend, and only stops when Jake spits him out. He is then immediately dismissive of Jake's complaints, and remains obstinate about his opinion until the very end of the episode, when he reenters Jake's body and sends both of them falling into a volcano, massively injuring them both. Yet another example of Finn's disdain for anyone else's opinions and methods.
  • "Frost & Fire": After experiencing a delightful dream as the result of watching Flame Princess and Ice King fight each other, Finn beings to manipulate events so that he can have the dream again and finish it. He writes hurtful notes to both Flame Princess and Ice King to rile them both up, using their private information against them, which culminates in a battle that destroys the Ice Kingdom. This is a major example of Finn's manipulative behavior for his own benefit, showing very clearly that he does not care about hurting people, as long as he achieves his own ends.
  • "Box Prince": When Finn comes across the lost prince of the Box Kingdom, he attempts to reveal the usurper of the throne and give the real prince his kingdom back. However, the entirety of the events involving the Box Kingdom appear to be just a bunch of cats doing cat-related things, making Finn's efforts seem pointless and silly. He even questions his own actions, asking himself, "Is this even a kingdom...?". This indicates that Finn will create a situation of conflict, even if there appears to be none, another example of his manipulative personality.

Self-harm (Repression/Masochistic Behavior)

(This particular behavior is not very apparent until much later in the series. Each entry is marked as "Physical", "Mental", or "Emotional" to differentiate.)

  • "Memories of Boom Boom Mountain" - Emotional: The memory of Finn being abandoned in the woods as a baby, then defecating onto a leaf and becoming stuck on it causes him to cry when he poops, as stated by Jake. While this may not be considered self-harm in the traditional sense, it does show that Finn retains deep psychological damage from his abandonment, and may have contributed to his development of BPD in the first place (see Possible Influences below)
  • "Morituri Te Salutamus" - Physical: While fighting the Gladiator Ghosts, Finn seems to delight in being punched in the face by one of them.
  • "The Creeps" - Mental: After encountering the Ghost Lady while escaping from the fake ghost, Finn locks the memory of her away in what he calls 'the vault'. This is a classic example of memory repression, or dissociation, which is a common attribute of BPD sufferers.
  • "Five Short Graybles" - Physical: Finn claims that he "likes the pain" of a long distance high five, and sets up an elaborate plan to make the biggest high five he possibly can.
  • "Davey" - Mental & Physical: In order to hide himself from his fans, Finn creates a new persona named "Davey Johnson". While this may not be self-harm, exactly, it can be seen as another form of repression, due to the fact that Finn does not deal with his marauding fans in any significant way, instead choosing to cut himself off from his immediate problem entirely. In addition, he shaves off most of his hair, which is considered physical self-harm.
  • "Puhoy" - Mental: After Flame Princess doesn't laugh at Finn's joke, he chooses to "fester" on his thoughts for a while inside the Pillow Fort, instead of actually talking to Flame Princess and clarifying the situation.
  • "Jake Suit" - Physical: Finn spends almost the entire episode taking physical and emotional pain while Jake controls his body. He is unaffected by being touched and licked by Fire Wolves and seems to enjoy having his head dipped inside a volcano. Even after he proved his point to Jake, he jumped back into the volcano for no reason, greatly injuring them both. This is a major example of Finn's masochistic behavior.
  • "The Vault" - Mental: This episode marks the first time when Finn's repressed memories affect his life. He also shows complete refusal at the idea of having to open his 'vault' in order to deal with his problem, and appears to display dissociation after having an episode (his voice becomes lifeless and his thoughts become distracted immediately following his nightmare - this is a classic example of dissociation)
  • "Dungeon Train" - Physical, Mental and Emotional: Rather than deal with his mounting emotional distress following his breakup with Flame Princess, Finn opts to spend the rest of his life battling on the Dungeon Train because "stuff makes sense" there. He appears to prefer physical pain over emotional pain, as he expresses delight in having acid spit on his unprotected leg, and seems to think that spending his life on the train is a much better solution to his problems - another clear example of repression. In addition, at the beginning of the episode, Finn displays what may be another instance of dissociation, as he appears to be distracted, vague, and lifeless (saying odd things such as "What is the meaning of soup?" and "What is the meaning of spoon?"), and says that his inside voice is "quiet".
  • "James" - Emotional: At the start of the episode, Finn tries to tell Jake to stop showing his sadness by telling him to "play it off... Make a normal face." This hints that Finn has experience in hiding his sadness from others.
  • "Rattleballs" - Physical: Finn seems to not mind sitting on hot coals, screaming that he "can take it", as well as allowing Rattleballs to cut out his eyeball. Another example of his masochistic tendencies.
  • "The Tower" - Emotional, Mental, and Physical: Following the events of "Escape From the Citadel", Finn appears to almost constantly dissociate with basically everything, which eventually culminates in him manifesting an 'electro-emotional prosthetic" arm in order to take revenge on his father. His voice is very lifeless throughout the episode, and while he builds his 'tower of revenge' he sings about tearing off his father's arm, which is an indication of just how deeply the events of "Escape From the Citadel" affected him. After telepathically tearing pieces out of nearly everything in the surrounding area, Finn builds his tower so high that he begins to asphyxiate, and he would have died if Princess Bubblegum had not saved him. This episode marks the most stark indication of Finn's emotional damage, and his attitude and reactions closely correlate with aspects of Borderline Personality Disorder.
  • "Breezy" - Emotional & Mental: Finn continues to display his near-constant dissociation, claiming that he "feels nothing" and that he is "lost in the darkness". He cannot remember what love feels like, which causes his flower arm to wilt, making him lock the memory in his 'vault' once again. In addition, Finn suggests making out with a bunch of princesses to reach a "good vibe saturation point" in order to "feel something" again, another example of his dissociation, and his willingness to bounce from one relationship to the next (see topic above).

Possible Causes

(While genetics are a large factor in determining the likelihood of developing BPD, there is no way to prove that Finn is more likely to be a sufferer based solely on his genetics.)

The largest contributing factor to Finn's possible BPD is shown in "Memories of Boom Boom Mountain". He is shown (in his memory) to have been abandoned in the forest at a very young age and was ignored by everyone who passed him until Joshua and Margaret adopted him. Loss of caregivers is a very strong contribution to many sufferers of BPD, as is chronic early maltreatment (Finn was lying on his own feces for an entire day). Such trauma could very much attribute to his development of a personality disorder.

Another, more subtle cause of the disorder may be a loss of identity. Finn was raised by dogs, and has never met any other humans that were not altered in some way (this does not include the existence of Susan Strong, as her species has yet to be confirmed). Therefore, Finn did not receive the proper "human" social cues and teachings during his early development, making the lack of a real identity a possible factor in his development of BPD.

Futhermore, an additional cause of Finn's BPD could be that his father Martin also has the disorder. While this can be considered speculation due to the short amount of time he has been featured on the show, Martin also displays behavior that correlates very closely with common BPD symptoms. He is shown to be extremely manipulative purely for his own benefit (such as when the flesh on his leg is seared off; he manipulates Finn in order to have him retrieve Guardian blood to heal his leg, and he is able to instantly become the de facto leader of the escaping cosmic criminals in order to escape from his son), disinterested, and generally only concerned about his own wellbeing. Borderline Personality Disorder can be a hereditary disorder (though typically environmental influences are more common in contributing to the development of the disorder), so if Martin is indeed a sufferer of BPD, then the likelihood of his son being genetically pre-dispositioned to be a sufferer is very high.

Additional Notes

(This section may contain minor speculation, based on unspoken or visual information)

While it cannot be said that this aspect of Finn's personality was decided upon in advance, it does seem like many of his actions throughout the series' continuing run correlate with one another. In addition, Borderline Personality Disorder generally begins to manifest itself significantly during adolesence. Since Finn is the only character who ages consistently with the series, it would make sense that only recently have the signs become much more apparent. He has dealt with a lot during the past couple seasons, and if his personality disorder is developing alongside the series' storylines, it stands to reason that his age would be a factor in the development of his BPD. Along with his aging, Finn shows to be less inhibited in showing off his "human" side as the series progresses, such as with the removing of his hat, which did not even happen until the Season 2 episode "To Cut a Woman's Hair", and revealing that he has deuteranopia, a form of ocular dichromatism that makes him unable to see the color green properly (he states that green things appear as "a dark grey-ish red" in "Red Starved"). Therefore, if the show is capable of displaying Finn's humanity in a physical way, it does not seem so odd that they would also display it in a mental way, seeing as how he already suffered from intense thalassophobia until the events of "Billy's Bucket List". But, with him possibly being the only human left in Ooo (until the arrival of Betty Grof), it stands to reason that the studying of human psychology would have almost certainly faded out entirely, meaning that even if Finn were to articulate his thoughts and feelings, they may never be fully understood or treated.
Some may argue that Finn appears too happy to be a sufferer of any mental disorder, but sufferers of BPD are often "exceptionally idealistic, joyful, and loving". BPD sufferers feel emotions more deeply than others do, and the same goes for happiness. Once something happens to depress Finn, however, his depression in generally longer and deeper than normal, showing the intensity of his emotional spectrum.
Considering the format of the show, it is possible that some of Finn's hasty reactions and impulsive behavior are simply the result of trying to convey the story within the show's runtime. While this point is very valid, it is pointless to speculate on how Finn would make decisions and consider other people's opinions and feelings if the show had more "time" to do so, simply due to the fact that the information presented is in the 11-minute format. This means that Finn's actions within the show's storylines (while not being realistic in non-fictional expression) conform to what can be displayed in the short amount of time the creators have. Taking into consideration Finn's actual canonical depiction in the show with the 11-minute format, and his actions therein, it can be correlated to a personality disorder (such as with his reactions to rejection in "Incendium" and "Earth & Water").

Thanks for reading!