It’s also likely that these recent cannabinoid findings are a small part of the larger story that is not yet totally clear. For example, there is evidence that a genetic variation in the mu opioid receptor, the target of morphine, OxyContin and other opiates, has a protective effect against opiate addiction.

Still, does this cannabinoid mutation simply correlate with less anxiety, and less addiction to marijuana — or does it cause them?

To answer that question, Dr. Francis S. Lee, a professor of psychiatry, and Iva Dincheva, a researcher, both at Weill Cornell Medical College, along with colleagues at the University of Calgary and elsewhere, took the human FAAH variant gene and inserted it into mice, where they could see the gene in action and study its specific effects. They simultaneously studied a group of human subjects with the variant FAAH gene. (The study was published last week in Nature Communications.)

Sure enough, these “humanized” mice that got the variant gene were less anxious, as evidenced by their spending more time in the open section of a maze. (More anxious mice, in contrast, prefer the safety of the enclosed arms of the maze.) And, just like people with this same gene, they showed similar changes in the neural circuits involved in anxiety and fear. Specifically, they had greater connectivity between the prefrontal cortex, the executive control center, and the amygdala, which is critical to processing fear, than the animals with the normal FAAH gene. A stronger connection between these two brain regions is known to predict lower anxiety and greater emotional control.

The benefits of this cannabinoid mutation don’t stop there. When Dr. Lee fear-conditioned the mice and human subjects by teaching them to associate a previously neutral stimulus with a noxious one, like a noise or electric shock, all the subjects — regardless of the genetic variant — learned the fearful associations equally well.

But when he taught the same subjects that the previously dangerous cue was now safe, by repeatedly presenting this stimulus without the noxious one — a process called fear extinction — the results were startling. Both mice and humans with the cannabinoid mutation showed enhanced fear extinction — that is, they learned more efficiently how to be unafraid.

So it seems that nature has designed us all to be on high alert for danger: We all learn to be afraid of new threats with equal facility. But some of us, like those with this cannabinoid mutation, forget about previous dangers more easily and move around in the world with less anxiety. This seems like a good deal for the species: We’re protected by those who are anxious and vigilant and enriched by those who are more carefree and exploratory.