Think about what bone you sit on. It’s called the Ischial Tuberosity. It is at the base of your pelvis which provides the base for your spine to fit in. You can imagine if one side is pushed higher the effect it has on your pelvic alignment and spinal alignment. As your pelvis and spine are deformed with activities such as sitting on a wallet, ligaments holding your pelvis and spine together stretch. Do it long enough and the laxity of these ligaments can increase permanently, leading to chronic spinal or pelvic misalignment and instability.
Symptoms caused by this could be sciatic pain, glut and or lower back pain. Additionally this can effect the ability to exercise. Leading to a truckload of problems.
With the current use of cards that you tap, thick wallets should be a thing of the past. However even a thin wallet used consistently over time will create enough disturbance in the bones and joints of your lower back and pelvis.
Clinically we see pelvic instability a lot. Whether it be postnatally, injury related or degeneratively. It can be a real tough problem to fix with lots of work for the patient to do themselves. So the best medicine in this case is definitely prevention.