A recent article in the Huffington Post made me realize how society never talks about slouching posture as a cultural act of defiance beyond the teenage years. But grown-up artist Sarah Lucas does just that, and I like the artistic expression of it. If we don't talk about the devastating health effects of habitual slouching but look at it as an artistic standpoint, we get an interesting new perspective:

".....Defiant slumping dates back at least to 1913 with the “debutante slouch,” a self-conscious craze (documented around the time by the Library of Congress and The New York Times) used to describe women of all classes who walked with their “shoulders sloping, chest dropped, hips slung forward and the knees... slightly bent.” For many, the posture went hand in hand with women drinking, smoking and casting corsets aside; it was something to be medically advised against. For others, it was a signifier of imperfect change, of (mostly white) women finally harnessing defiance, vying for jobs in male-dominated fields and protesting for the right to vote, slouch-shaming be damned.

In recent years, celebrities like Kristen Stewart, Emma Watson and Keira Knightley have inspired tsk-tsking tabloid headlines for their refusal to assume classic starlet stances on carpets. And in case you needed any additional evidence that slouching is a good thing, Jordan Peterson advises against it. 
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As someone who has the posture of Eeyore holding a smartphone, Lucas’ devotion to slouch life hits me where it aches (in my lower spine). I have only felt shame in relation to my posture, perceiving it as a sign of my indefatigable fatigue and inability to function in even the most basic of ways. I’d never considered that a slouch could be something to be proud of, a way of wordlessly communicating some form of BDE...."

If the above mentioned actresses assumed the slouch habitually or made a conscious choice to make a cultural statement is still unclear.