So, it is more or less widely known that abilities are somehow connected to the user's soul and will. (Since we see Yosano's ability specifications change after the war.)
My personal theory is that abilities, much like coping mechanisms, are a way of the mind to take form in whatever it's user thinks will help them most. This would make abilities a direct parallel to trauma.
This would explain such abilities as beast beneath the moonlight or Anne's room. They are both, diffrent types of responses to early childhood trauma. While Atsushi wanted protection from something stronger then himself, that can keep him safe and shoulder the memoriesof his abuse. This reminds me of dissociative disorders, mainly DID, since the tiger seems to have a mind of its own. Atsushis arc, very clearly centers around coming to terms with the abuse he faced, accepting and understanding the tiger that kept him face throughout.
Lucy on the other hand escaped into a playground in her own mind. A friend in her minds cape that she controls to keep her company. This displays both themes of escapism and lack of agency. Unlike the tiger, Anne doesn't seem to have its own will, instead being a direct extention of Lucy. Someone who can't hurt her because Lucy is in complete control in her own mind, unlike the outside world.
Similarly this is applicable to Yosano herself, as a child during war times with a strong need to help those around her, she developed an ability to keep the people around her safe. We don't know much about who Yosano was before being drafted by Mori, but since they crossed paths, it can be assumed that Yosano came into direct contact with the war, almost loosing those around her to it.
Teruko, also being a child in war times was drafted very young. We don't know if her ability developed before or after being involved in the war, but it can be assumed that it is quite litteraly a visualization of her growing up too fast and later in life displaying themes of age regression due to this fact.
Akutagawa had to keep himself and his family safe while having nothing to his name other than the clothes on his back. Being one of the oldest in his family group, it was on him to protect the others in a hostile environment, so he made the one thing he had into his weapon. (It also does not display defensive traits untill being shot at by dazai, suggesting akutagawa did not see himself as worth protecting before that)
Fyodor's ability is likely (without going into spoilers) stemming from a need to survive in spite of those wanting him dead. Perhaps tied to religion in some way aswell.
Tachihara's ability is an interesting one since it mimics the one his brother had. An ability that developed to live up to the constant comparison to his dead brother. A brilliant hero of war, that died despite it all. A memory he could never live up to, even if he mimicked the ability that made his brother into the hero he was.
Demon snow in this context can be classified as generational trauma. Perhaps it wasn't actually transfered from her mother to her, but instead she developed an ability mimicking that of her mother's since that was all she knew could protect her. At the same time she cannot control it in the beginning, because the trauma she bears wasn't hers to begin with. She does not understand it or know what it was made for, so she struggles with accepting it as her own.
Dazai's ability in this context is an odd one too. It is an ability to protect itself from other people's trauma. A trauma developed through others in a very direct way. Perhaps it is metaphoric, trying to distance himself from others and the human emotions that come with it. Perhaps very physical, if dazai had grown up in an environment where other abilities often paused a threat to him and his wellbeing. (This does tie into some other theories I have about him)
This explains also some of the more... odd abilities, like the lemon guy. Instead of finding out that he is immune to lemon bombs, he built a lemon bomb and when it went off, his ability adapted to keep him safe.
Now, just like trauma, some of the reasons for developing abilities are very clear due to the violet nature of their necessity. Other coping mechanisms are less obvious, unconscious even to the users themselves.
Fukudzawa's ability stems from the fear and need to protect those closest to him. Give them the ability to use their powers to the fullest while keeping them safe from the downsides, like he did with Ranpo.
Some abilities seem trivial in comparison, like Kunikida's or Kenji's abilities. They can be likened to simple convenience, but I suspect that while they may not have obvious causes like some others, there still are mental strains associated with them. Such as Kunikida's will to bring his ideas to reality or Kenji's need to be strong enough to help in the village (tied to his hunger perhaps for the reason that he needed to work to keep food on his plate).
Tanizaki's ability is directly tied to deceit. Making others see and perceive this differing from reality. This does tie into his and Naomi's relationship quite well, though there might be many reasons for it to have happened.
Mori's ability, that I suspect reflects his guilts and failings. Mimics all of that he could not do right by. Haunting him, forever unsatisfied for his efforts, while also being loyal to him in similar ways most the people he hurt were. Protecting him despite their hatred for him. (It is a hard one to understand, mainly because mori himself is so often lying about it and how it works)
Some traumas on the other hand leave no memories in the users mind. No hint of why the came to be or what their purpose is. Due to Chuuya's story it is quite unclear how exactly his ability developed and whether or not it changed aside from the singularity he can now cause. It is entirely possible that his original ability had a much more specific connotation that was artificially removed to make him more broadly applicable.
It is quite fitting for bsd, which ultimately is a story about how different people coap with trauma, to have its central magic system tied to exactly that. But I have seen very few people actually discussing the parallels or it and the implications they bring.
I can probably talk for hours about any ability in bsd and how it connects to this, so it anyone is interested in my ramblings, I can do deepdives lol.
//tldr: abilities show striking parallels to real life coping strategies usually caused by trauma.
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