Redheads may be hotheads, but they get colder quicker. They also bruise more easily. And they feel more pain.

All this comes from a series of studies done in the last few years on people with genes for red hair. Red hair is red due to excess production of the molecule pheomelanin. While we brunettes produce more eumelanin (dark brown pigment), redheads produce an excess of pheomelanin. Too much of this yellow-red pigment is the result of mutations in the MC1R.3 gene. Redheads have two copies of this variant allele, one from each parent.

So what does the “redhead gene” have to do with sensitivity? University of Louisville anesthesiologist Edwin Liem suspects that when both copies of the MC!R.3 have mutations, which is the case for redheads, the melanocortin 4 receptor is faulty, inefficient, and/or unable to be transported to the surface of cells. This receptor happens to modulate sensations of cold and pain. Another explanation is that the mutation in the MC1R gene, the same one that produces too much pheomelanin, affects hormones that stimulate pain receptors in the brain.

In one study, Liem and his colleagues compared the pain tolerance of sixty naturally red-haired volunteers with sixty brunettes. The redheads began to feel chilling pain at around 6C (43F), unlike the volunteers with dark hair, who did not really begin to ache until the temperature approached freezing. In another study, by Liem et al., women with various hair color types were exposed to electric shock. Turns out, the redheads needed about 20 percent more anesthetic to put down the pain (confirming the common belief among anesthesiologists that redheads are tough to knock out). While redheads have normal blood counts and coagulate blood the same as anyone else, a study found that they bruise more easily. Yet another study found that redheads are more than twice as likely as women with other hair colors to fear and avoid the dentist.

These studies have been done on women only, and it’s unknown whether red-haired men would have the same outcome. However, there’s evidence that pain pathways differ between the sexes.

Redheads are stereotyped as being hot-headed, tempestuous, dramatic, high-strung. Is it possible that a genetic sensitivity to pain can affect temperament? It’s fun to speculate. For some, physical pain may translate into emotional pain. Sensitivity may tip over into volatility. Could a fiery, short temper even be a pain avoidance mechanism? Why not — after all, a good offense can be the best defense.