By and large, the higher your vagal tone the better. It means your body is better able to regulate the internal systems that keep you healthy, like your cardiovascular, glucose and immune responses.

Beyond these health effects, the behavioral neuroscientist Stephen Porges has shown that vagal tone is central to things like facial expressivity and the ability to tune in to the frequency of the human voice. By increasing people’s vagal tone, we increase their capacity for connection, friendship and empathy.

In short, the more attuned to others you become, the healthier you become, and vice versa. This mutual influence also explains how a lack of positive social contact diminishes people. Your heart’s capacity for friendship also obeys the biological law of “use it or lose it.” If you don’t regularly exercise your ability to connect face to face, you’ll eventually find yourself lacking some of the basic biological capacity to do so.

The human body — and thereby our human potential — is far more plastic or amenable to change than most of us realize. The new field of social genomics, made possible by the sequencing of the human genome, tells us that the ways our and our children’s genes are expressed at the cellular level is plastic, too, responsive to habitual experiences and actions.

Work in social genomics reveals that our personal histories of social connection or loneliness, for instance, alter how our genes are expressed within the cells of our immune system. New parents may need to worry less about genetic testing and more about how their own actions — like texting while breast-feeding or otherwise paying more attention to their phone than their child — leave life-limiting fingerprints on their and their children’s gene expression.

When you share a smile or laugh with someone face to face, a discernible synchrony emerges between you, as your gestures and biochemistries, even your respective neural firings, come to mirror each other. It’s micro-moments like these, in which a wave of good feeling rolls through two brains and bodies at once, that build your capacity to empathize as well as to improve your health.

If you don’t regularly exercise this capacity, it withers. Lucky for us, connecting with others does good and feels good, and opportunities to do so abound.

So the next time you see a friend, or a child, spending too much of their day facing a screen, extend a hand and invite him back to the world of real social encounters. You’ll not only build up his health and empathic skills, but yours as well. Friends don’t let friends lose their capacity for humanity.