I notice when I look in the mirror that one shoulder is always much higher than the other one. What causes this?

Differences from one side of the body to the other side are common. In fact, photos taken of the face comparing one side to the other show that we aren't really symmetrical (the same from side to side) at all. Minor differences aren't usually very obvious. So, if you can see a big difference from one side to the other, there may be other factors involved.

For example, hand dominance can play a part in asymmetries (not symmetrical; uneven). People who are strongly dominant on one side (e.g., very right-handed) use that arm much more than the other side. This can lead to larger muscles on one side and differences in alignment. That may be what you are observing in the mirror.

Another possibility is spinal alignment. Even a small degree of scoliosis (abnormal curvature of the spine) can create differences in the shoulders and/or hips. The tops of the shoulders appear uneven. The hip is higher on one side than the other. The difference in hip alignment is most noticeable in pants length (one side always appears longer than the other).

A third, common cause of shoulder asymmetry is scapular position. The scapula is the shoulder blade. The scapulae (plural) are normally positioned on either side of the spine between the second and seventh thoracic vertebrae. Elevation or depression of the scapula can also give the appearance of a shoulder that's higher on one side.

If you are concerned that there may be something going on that requires medical attention, see your doctor. A physical therapist can also assess your posture and alignment. There may be some simple postural exercises that can bring you back into a more normal, neutral position.