Does bad posture cause back and neck pain?

Short answer

No. Spine shape and a bent neck while using a phone were not linked to pain, and people who slouched at 17 were not in more pain 5 years later. What raised pain was sitting for a long time, not the shape of the posture.

Does bad posture cause back and neck pain?
'Slouching → strain on the spine → pain' — is that right?
A common belief says back and neck pain come from bad posture. We checked whether spine shape and pain really go together, whether bad posture leads to future pain, and what actually raises pain.
Spine shape and pain were separate
54 studies found no strong link
A review of 54 studies on the front-to-back curves of the spine and health found no strong link between spine shape and back or neck pain. Of 35 reports on sitting at work, all but 1 found no link with low back pain.
The same for a bent neck on your phone
Neck angle was not linked to neck pain
When 582 people's neck angle was measured while texting on a phone, the angle, standing or sitting, was not linked to whether they had neck pain, how often, or how bad. When 1,108 people's sitting neck posture was photographed and sorted into 4 types, neck pain and headaches did not differ between the groups.
Adults with neck pain held their heads further forward
But which came first could not be told
A review of 15 studies pooled 3 that compared 203 adults: those with neck pain held their heads further forward. But all 15 measured posture and pain at the same moment, so they cannot tell whether posture caused the pain or pain changed the posture.
Following people for 5 years
Those who slouched were not in more pain
686 people were grouped by sitting neck posture at 17 and asked about ongoing neck pain at 22. In men, posture made no difference; in women, the slouched group actually had 0.24 times the risk of the upright group. The best predictor of pain at 22 was neck pain already present at 17 (3.78 times).
The problem is 'long', not the shape
Sitting a long time raised back pain on the spot
Of 10 studies that measured sitting time with devices, 9 found back pain rose on the spot the longer people sat. Standing was not the answer either: pooling 3 studies, standing for long periods at a desk did not lower back pain compared with sitting.
Bottom line: posture shape didn't create pain
Staying in one position for a long time was the problem
Spine shape and a bent neck were not linked to pain, and people who slouched were not in more pain 5 years later. What raised pain was sitting for a long time, and standing for a long time was no better. Read it as: not staying in one position too long matters more than holding a perfect posture.