There are many theories about the basic types of human emotions. Paul Ekman identified six universal basic emotions: happiness, sadness, disgust, fear, surprise and anger. Robert Plutchik proposed emotions can be combined like colors. A 2017 study identified 27 basic emotion categories that people experience along a gradient. Happiness is characterized by joy and well-being while sadness involves disappointment. Fear triggers fight or flight responses. Disgust involves repulsion and anger feelings of hostility. Surprise elicits a startling response.