An individual's characteristic emotional nature, including energy level, prevailing mood and sensitivity to stimulation.

Individual variations in temperament are most readily observed in newborn babies. Even immediately after birth, some babies are calm while others cry a lot. Some respond favorably to being held while others squirm and protest. Some are soothed by soft music and others do not stop crying long enough to hear it. Because of these immediately observable variations, temperament is often considered a biologically based characteristic.

Hippocrates discussed variations in temperament as early as the 5th century B.C. His hypothesis that there are four basic human temperaments that correspond to various bodily characteristics—choleric, sanguine, melancholic, and phlegmatic— endured for many years before modern theories...