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Ingrown toenails basically don't happen in cultures where people don't wear shoes.

I suspect the cause is socks or shoes pushing against the skin at the end of the toe, causing it to grow incorrectly over many months.

An untrimmed nail pushes any sock/shoe away, solving the issue.

It keeps the side from curling in and causing the irritation/cut that results in what we call ingrown toenail.

The nail doesn't change, rather the skin by the end of your toe does.