In medieval Europe and early colonial America, moles were seen as a sign of demonic possession (as was everything else). The ancient Greeks and Chinese, however, used the markings to try to decipher personality traits or destinies. In the age of modern pop culture, icons like Marilyn Monroe, Goldie Hawn, Cindy Crawford, and Madonna have turned moles into beauty marks to be desired and even emulated. But as anyone whose skin is accentuated by a mole can tell you, there is something particularly unique about them: They seem to grow hair at a faster rate than ordinary skin, and that hair often seems darker and coarser.

Why is this? Why do hairs burst forth from moles more quickly than they do from ordinary skin? Science actually doesn’t know.

“The short answer is that we’re not sure why the hair is coarser in moles or why it may seem to grow faster,” Lauren Ploch, a dermatologist based in Augusta, Georgia, tells mental_floss. “The development of moles is still a dermatologic mystery, as is the regulation of hair growth.”

When viewed under a microscope, nevus cells, a kind of melanocyte found in moles, don't invade the structure of hair follicles and seem to have no effect that would change hair appearance or growth, says Ploch.

Researchers do know that mole skin is different from other skin and develops as the result of a number of factors, including genetics, environment, hormones, and signaling proteins. Many of these elements (especially hormones and signaling proteins) also contribute to hair growth, says Ploch. So the presence of the mole and the underlining causes of it may tinker with the usual bodily factors regulating hair growth.

“I suspect that, while the mole itself may not have a direct role in creating a darker, coarser hair, the local milieu of signaling molecules and hormones in the skin that created the mole leads to a darker, coarser hair within the lesion,” she says.

She adds that moles often develop during puberty in response to androgen, the same steroid hormone that spurs facial and pubic hair growth, so moles and hair have a common stimulus. 

Science has yet to fully explain the behavior of moles, but at least we know more about them than we did during the middle ages.