Mostly internal thoughts affected by a negative loop that an external situation triggers. The maintenance of the loop is also physical. Reaction by the nervous system after enough repetitions. The statistically ever-increasing bent spines are a result of them being a loop of muscle adjustments in various parts of the body, but prominently also seen around the neck. Minds have evolved(de) to react to the normal straight spine - "Why are you so stiff... relax?; bend your spine." The muscles in and around your neck adjust to the pressure on your spine making it feel natural in most circumstances. Shallow breaths, caused by slouching more, leading to the brain being in a constant state of danger, makes it easy for negativity to take over. The protective stance physically closes the heart and throat in response to negativity.