THE link between mind and body is terrain into which many medical researchers, fearing ridicule, dare not tread. But perhaps more should do so. For centuries, doctors have recognised the placebo effect, in which the illusion of treatment, such as pills without an active ingredient, produces real medical benefits. More recently, respectable research has demonstrated that those who frequently experience positive emotions live longer and healthier lives. They have fewer heart attacks, for example, and fewer colds too.

Why this happens, though, is only slowly becoming understood. What is needed is an experiment that points out specific and measurable ways in which such emotions alter an individual’s biology. And a study published in Psychological Science, by Barbara Fredrickson and Bethany Kok at the University of North Carolina at Chapel Hill, does precisely that.

Dr Fredrickson and Dr Kok concentrated their attentions on the vagus nerve. This nerve (illustrated right, in an early anatomical drawing) starts in the brain and runs, via numerous branches, to several thoracic and abdominal organs including the heart. Among its jobs is to send signals telling that organ to slow down during moments of calm and safety.

How effectively the vagus nerve is working can be tracked by monitoring someone’s heart rate as he breathes in and out. Healthy vagal function is reflected in a subtle increase in heart rate while breathing in and a subtle decrease while breathing out. The difference yields an index of vagal tone, and the value of this index is known to be connected with health. Low values are, for example, linked to inflammation and heart attacks.

What particularly interested Dr Fredrickson and Dr Kok was recent work that showed something else about the vagal-tone index: people with high tone are better than those with low at stopping bad feelings getting overblown. They also show more positive emotions in general. This may provide the missing link between emotional well-being and physical health. In particular, the two researchers found, during a preliminary study they carried out in 2010, that the vagal-tone values of those who experience positive emotions over a period of time go up. This left them wondering whether positive emotions and vagal tone drive one another in a virtuous spiral. They therefore conducted an experiment on 65 of the university’s staff, to try to find out.

They measured all of their volunteers’ vagal tones at the beginning of the experiment and at its conclusion nine weeks later. In between, the volunteers were asked to go each evening to a website especially designed for the purpose, and rate their most powerful emotional experiences that day. Dr Fredrickson and Dr Kok asked their volunteers to consider nine positive emotions, such as hope, joy and love, and 11 negative ones, including anger, boredom and disgust. They were asked to rate, on a five-point scale, whether—and how strongly—they had felt each emotion. One point meant “not at all”; five meant “extremely”. In addition, half the participants, chosen at random, were invited to a series of workshops run by a licensed therapist, to learn a meditation technique intended to engender in the meditator a feeling of goodwill towards both himself and others. This group was encouraged to meditate daily, and to report the time they spent doing so.

Dr Fredrickson and Dr Kok discovered that vagal tone increased significantly in people who meditated, and hardly at all in those who did not. Among meditators, those who started the experiment with the highest vagal-tone scores reported the biggest increases in positive emotions. Meditators who started with particularly low scores showed virtually no such boost.

Taken as a whole, these findings suggest high vagal tone makes it easier to generate positive emotions and that this, in turn, drives vagal tone still higher. That is both literally and metaphorically a positive feedback loop. Which is good news for the emotionally positive, but bad for the emotionally negative, for it implies that those who most need a psychosomatic boost are incapable of generating one. A further (as yet unpublished) experiment by Dr Kok suggests, however, that the grumpy need not give up all hope. A simpler procedure than meditation, namely reflecting at night on the day’s social connections, did seem to cause some improvement to their vagal tone. This might allow even those with a negative outlook on life to “bootstrap” their way to a mental state from which they could then advance to the more powerful technique of meditation.

Whether, besides improving general health, the mechanism Dr Fredrickson and Dr Kok have discovered helps explain the placebo effect remains to be investigated. But it might, because part of that effect seems to be the good feeling engendered by the fact of being treated. More generally, doctors in the ancient world had a saying: “a healthy mind in a healthy body”. This sort of work suggests that though this proverb is true, a better one might be, “a healthy mind for a healthy body”.