Training Institute for Posture and Movement © Un site par Axxun
A new research article investigated the link between "sitting upright" (a concept to be more clearly defined) and psychological state:
“Compared to sitting in a slumped position, sitting upright can make you feel more proud after a success, increase your persistence at an unsolvable task, and make you feel more confident in your thoughts,” Dr. Broadbent explained. "Research also suggests that sitting upright can make you feel more alert and enthusiastic, feel less fearful, and have higher self-esteem after a stressful task.”
The clinical practice indeed often shows a strong psychological impact of posture therapy. [...]
In turn, this positive impact on the psyche plays a role on the patient's relationships with his social environment, which helps the treatment too: being in a good mood and showing a positive body language helps having great relationship, which are good for health!
On the same topic, have a look at Amy Cuddy's TED Talk.