Human beings are essentially biologically hard wired to monitor and detect fellow humans' emotions, particularly in the face. Of course, some people are better equipped to pick up on emotional cues than others. The human process of detecting and reacting to emotions has been called empathy and sympathy. These facilities were necessary in early civilizations to ensure the survival of the species. Many anthropologists believe early communication was non-verbal and thus communication was something akin to a mixture of telepathic communication and biological response. Therefore, the ability to transmute emotional data to other people is vital to human survival.
As some human beings moved from large tribal communal structures to smaller family units, the necessity of reading the emotions of others did not diminish but was rather more localized to those close to an individual. Reading the needs and emotional tone of family members helped ensure the family lineage remained intact. This includes reading the spouse and children for distress. Most parents can relate to the ability of detecting children's sadness, anger, frustration or happiness and will respond accordingly. In fact, one of the early ways in which human beings learn to respond to emotional content is through first family relationships.
Fear is one of our most primitive human emotions and is thought to be the sole emotion responsible for our early survival. Human beings had to know when to engage a fight-or-flight response to a threat, and thus fear was developed in order to preserve the organism. Learning to detect fear in yourself can save you from dangerous situations. Conversely, detecting fear in others can help you to respond accordingly to allay those fears if necessary. Fear is one of the more virulent social emotions and is easily transferable due to its connection to the survival of the organism.
Many social psychology researchers narrow many of the emotions down to two basic emotions, love and fear, stating that all other emotions branch out of those two primary emotional responses. Love is an emotional response that also ensured the survival of the species as it helped people to detect interest in potential mates. As humans evolved, love expanded to include a variety of emotions including joy, excitement and enthusiasm. Love is an infectious social emotion, and thus when someone is genuinely happy and near others it can be detected and transmuted in the same way fear is transmuted. Understanding the principles of social emotional transmission can help you be more cognizant of what emotional messages you are sending to the community.