Emotionality is the observable behavioral and physiological component of emotion, and is a measure of a person's emotional reactivity to a stimulus. Most of these responses can be observed by other people, while some emotional responses can only be observed by the person experiencing them. Observable responses to emotion (i.e., smiling) do not have a single meaning. A smile can be used to express happiness or anxiety, a frown can communicate sadness or anger, and so on. Emotionality is often used by psychology researchers to operationalize emotion in research studies.