The greater degree to which a persons pupils mirror another predicts a persons greater score on empathy. In disorders such as autism and psychopathy, facial expressions that represent sadness may be subtle, which may show a need for a more non-linguistic situation to affect their level of empathy. Vocal expression
According to DIPR scientist Swati Johar,: VII  sadness is an emotion "identified by current speech dialogue and processing systems". : 12  Measurements to distinguish sadness from other emotions in the human voice include root mean square (RMS) energy, inter-word silence and speaking rate. It is communicated mostly by lowering the mean and variability of the fundamental frequency (f0), besides being associated with lower vocal intensity, and with decreases in f0 over time. Johar argues that, "when someone is sad, slow, low pitched speech with weak high audio frequency energy is produced". Likewise, "low energy state of sadness attributes to slow tempo, lower speech rate and mean pitch". : 10, 13 Sadness is, as stated by Klaus Scherer, one of the "best-recognized emotions in the human voice", although its "generally somewhat lower than that of facial expression". In a study by Scherer, it was found that in Western countries sadness had 79% of accuracy for facial recognition and 71% for vocal, while in Non-Western countries the results were of 74% and 58%, respectively.
Sadness is an emotional pain associated with, or characterized by, feelings of disadvantage, loss, despair, grief, helplessness, disappointment and sorrow. An individual experiencing sadness may become quiet or lethargic, and withdraw themselves from others. An example of severe sadness is depression, a mood which can be brought on by major depressive disorder or persistent depressive disorder. Crying can be an indication of sadness.Sadness is one of the six basic emotions described by Paul Ekman, along with happiness, anger, surprise, fear, and disgust. : 271–4

Childhood
Sadness is a common experience in childhood. Sometimes, sadness can lead to depression. Some families may have a (conscious or unconscious) rule that sadness is "not allowed", but Robin Skynner has suggested that this may cause problems, arguing that with sadness "screened off", people can become shallow and manic. : 33, 36  Pediatrician T. Berry Brazelton suggests that acknowledging sadness can make it easier for families to address more serious emotional problems. : 46, 48 Sadness is part of the normal process of the child separating from an early symbiosis with the mother and becoming more independent. Every time a child separates a little more, he or she will have to cope with a small loss. If the mother cannot allow the minor distress involved, the child may never learn how to deal with sadness by themselves.