Sadness is one of the seven basic emotions that cause human beings to make a predictable facial expression. If you are sad, you will make a sad facial expression, whether you want to or not. Even if you try to hide your sadness, the sad expression is likely to flicker across your face in a fraction of a second. Psychologist Paul Ekman calls these fleeting expressions microexpressions. Here’s a video that shows how to detect the microexpressions of sadness:

Sadness is a response to loss. Since everyone suffers loss at some point or another, sadness is a normal and inevitable part of life. Yet feelings of sadness can also be a symptom of a mental disorder called depression. What is the difference between ordinary sadness and clinical depression? As Dr. Allen J. Frances explains in the following video, even psychologists and psychiatrists disagree on how to draw that line.

Nevertheless, it is clear that depression is a serious illness. It is often deadly. In this video, biologist Robert Sapolsky explains just how serious depression is:

Depression is particularly common among people who write for a living. I have no idea why. As a result, you can find many literary descriptions of the experience of depression. One of the most useful is this blog post by cartoonist Allie Brosh. As she explains, sadness is part of depression. Yet depression can go far beyond that. As she put it, “I just drifted around, completely unsure of what I was feeling or whether I could actually feel anything at all.” She also described how this difficulty to have normal emotions made it hard for her to communicate with other people:

However, I could no longer rely on genuine emotion to generate facial expressions, and when you have to spend every social interaction consciously manipulating your face into shapes that are only approximately the right ones, alienating people is inevitable.

Depression has psychological effects, but the cause of the problem may be at least partly biological. Depression, including bipolar disorder, tends to run in families, which suggests that genes may play a role. Lifestyle factors may also play a role. In this video, Dr. John McDougall explains how exercise, high-carb diets, and the avoidance of oversleeping can help prevent depression.