What causes moles?

Mole causes: Nevi or "moles" are caused by abnormal proliferation of skin cells. This is technically a neoplasm, but is of the benign variety. Most appear in the first thirty years of life and can have a hereditary component.

They just happen: They actually result from a couple of mutations happening in a single cell of the nerve sheath family -- schwann cell or actual melanocyte depending on the kind of mole. These lesions are subclassified into lentigos, several types of common nevi, and a bunch of others. Common nevi and lentigos are more common if you got sunburns as a kid. There's a genetic component. Watch your nevi closely.