Asked about her mental state, she wrote: “No, I have never experienced anxiety. I have always been content and happy.”

How could a genetic mutation wipe out anxiety?

Dr. Cox said he believed that Ms. Cameron’s reduced anxiety was “related to increased signaling at CB1 receptors,” or cannabinoid receptors, which are known to help the body deal with stressful situations. (Notably, they are activated by the THC in cannabis.)

Block the cannabinoid receptors and anxiety will increase; boost the cannabinoid receptors and anxiety will fall, studies have shown. The receptors also affect how people experience physical pain.

Does that mean physical and mental pain are processed the same way?

No, it’s more complicated than that and lots of research is still needed, said Dr. T.H. Eric Bui of the Center for Anxiety and Traumatic Stress Disorders and Complicated Grief Program at Massachusetts General Hospital. What we do know, he said, is that “brain regions that process emotional and physical pain overlap.”

In another example of how mysteriously intertwined the two types of pain can be can be, he noted that acetaminophen (the active ingredient in Tylenol, among other pain relievers) had been shown to decrease the emotional pain that comes with rejection.

So is rejection similar to physical pain?

Naomi Eisenberger, a professor in the University of California, Los Angeles, psychology department, believes so. Dr. Eisenberger studies the similarities in the way that the brain processes physical pain and the “social pain” that results from rejection.

She said she had repeatedly found that “people who are more sensitive to physical pain are more upset by rejection.”