The most fundamental discovery of this new science: We are wired to connect.

Neuroscience has discovered that our brain’s very design makes it sociable, inexorably drawn into an intimate brain-to-brain linkup whenever we engage with another person. That neural bridge lets us impact the brain—and so the body—of everyone we interact with, just as they do us.

Even our most routine encounters act as regulators in the brain, priming emotions in us, some desirable, others not. The more strongly connected we are with someone emotionally, the greater the mutual force. The most potent exchanges occur with those people with whom we spend the greatest amount of time day in and day out, year after year—particularly those we care about the most.

During these neural linkups, our brains engage in an emotional tango, a dance of feelings. Our social interactions operate as modulators, something like interpersonal thermostats that continually reset key aspects of our brain function as they orchestrate our emotions.

The resulting feelings have far-reaching consequences, in turn rippling throughout our body, sending out cascades of hormones that regulate biological systems from our heart to immune cells. Perhaps most astonishing, science now tracks connections between the most stressful relationships and the very operation of specific genes that regulate the immune system.

To a surprising extent, then, our relationships mold not just our experience, but our biology. The brain-to-brain link allows our strongest relationships to shape us in ways as benign as whether we laugh at the same jokes or as profound as which genes are (or are not) activated in t-cells, the immune system’s foot soldiers in the constant battle against invading bacteria and viruses.

That represents a double-edged sword: nourishing relationships have a beneficial impact on our health, while toxic ones can act like slow poison in our bodies.

Virtually all the major scientific discoveries I draw on in this volume have emerged since Emotional Intelligence appeared in 1995, and they continue to surface at a quickening pace. I intend this book to be a companion volume to Emotional Intelligence, exploring the same terrain of human life from a different vantage point, one that allows a wider swath of understanding of our personal world.

When I wrote Emotional Intelligence, my focus was on a crucial set of human capacities within an individual, the ability to manage our own emotions and our inner potential for positive relationships. Here the picture enlarges beyond a one-person psychology—those capacities an individual has within—to a two-person psychology: what transpires as we connect.

Take, for example, empathy, the sensing of another person’s feelings that allows rapport. Empathy is an individual ability, one that resides inside the person. But rapport only arises between people, as a property that emerges from their interaction. Here the spotlight shifts to those ephemeral moments that emerge as we interact. These take on deep consequence as we realize how, through their sum total, we create one another.

— From the prologue to Social Intelligence