New research, published Friday, reveals why redheads may have altered sensitivity to certain kinds of pain.

The study, conducted by investigators at Massachusetts General Hospital (MGH) and published by Science Advances, suggests that cells that determine skin color — melanocytes — and certain factors involved, help decide how people experience pain.

Previous work has suggested that the inability of redheads to tan or darken their skin pigment is traced to inactive variants of the melanocyte receptor called melanocortin 1.

Now, research has found that the receptors also reduce the production of POMC, a chemical that causes lower levels of hormones, creating an equilibrium between pain-inhibiting and pain-enhancing receptors. This, along with additional factors that activate opioid receptors involved in blocking pain, give those with red hair a higher pain tolerance. The study was done on a strain of red-haired mice which lacked the melanocortin 1 receptor function. 

“These findings describe the mechanistic basis behind earlier evidence suggesting varied pain thresholds in different pigmentation backgrounds,” says David Fisher, director of the Mass General Cancer Center’s Melanoma Program in a statement. “Understanding this mechanism provides validation of this earlier evidence and a valuable recognition for medical personnel when caring for patients whose pain sensitivities may vary.”