Imagine if a person told you they were sitting on a 2 inch wedge for the last 20 years. Would you wonder why they may be having some problems with their spine? Many men put their wallet in their back pocket and then sit on it for 6,7,8 hours a day. This creates an imbalance in the pelvis, which leads to an imbalance in the spine.
Quick anatomy review. Your spine should be straight when looking at it from either the front or the back. If we sit on an uneven surface, read wallet, the spine must bend to compensate for the unevenness. Do this for an hour here and there and the effect is minimal; do this for 6-8 hours a day for many years, the problem is significant.
A bent spine places pressure on the nerves and muscles, leading to pain and stiffness just for starters. How about the organs that are connected to these nerves, organs like the kidney or bladder. Is it possible that sitting on a wallet for a long period of time could place stress on the nerves that control the kidney? I see it everyday.
The simplest solution is to either move the wallet to your front pocket, maybe a shirt pocket, or a suitcoat. If you insist on sitting on a wallet go for the thinnest one you can find. This wallet here gets great reviews.