The theme of emotional contagion is prevalent throughout the text of Social Intelligence. Emotional contagion is the phenomenon in which human emotions “spread” unintentionally from one person to another, just as a virus spreads through person-to-person contact. According to Goleman, emotional contagion factors into almost every aspect of human social experience. From anecdotes about “catching” another person’s fear or anger from only fleeting interactions to the hard science of experiments using MRI to show the emotional centers of the brain unconsciously mirroring the emotions it witnesses, emotional contagion forms the cornerstone of Goleman’s theories about social intelligence.

Emotional contagion can be seen as the driving force behind a wide range of human interaction, as both a source of warmth and a cause of conflict. Within couples, emotional contagion creates rapport and even leads to the partners starting to look alike as they grow old together since their mirror neurons match each other’s expressions and gestures so frequently, shaping their facial muscles accordingly. At the same time, one person’s rancor can spread to their partner just as unconsciously. Couples in bad relationships spread their unhappiness to one another, actually shortening their lifespans and making them more vulnerable to bad health.