A myth is a story (either fictional or partially factual) which is used to explain a natural phenomenon or to illustrate a moral or cautionary lesson.
If you go along to your doctor complaining of depression there is a fair chance that you will hear about serotonin. Perhaps the older term "chemical imbalance in the brain" will get a mention too. The idea is that the low levels of serotonin in the brain cause the symptoms of depression. Serotonin is a neurotransmitter which means that it is a chemical that nerve cells use to message one another. If it is in short supply then the brain nerve cells that are dependent on it will not function very well. Many of those cells help to maintain happiness, so when they fail we become unhappy. And if we can just give the serotonin a boost all will be well again. It is a good story - simple, easy to understand, seemingly scientific. But it is a myth....
There is evidence that serotonin is somehow linked to depression, not the least of which is the fact that most of the drugs which help depression do appear to boost serotonin levels. However the truth of the matter is that doctors do not really fully understand the neurochemical basis of depression or indeed most of human behaviour.
Some people object to using a neurochemical model to explain human feelings and behavior because they believe it denies the reality of free will, and that idea challenges some very important ideas regarding morality, ethics and religious belief. Humans clearly do not function as puppets with the strings being pulled by chemicals such as serotonin. Free will is something we all experience and use most of the time. But of course the operation of free will is limited by biological factors. We all have the choice of eating or fasting but most of us could not voluntarily maintain a total fast for much longer than a day, mainly due to the biological drive to eat, which eventually becomes stronger than our willpower. People with drug addiction have a similar problem - while the decision to use drugs is ascribed to a weakness of willpower the biological drive is often more powerful than the will to eat!
In depression a person's ability to make choices is hampered by the biological factors of their illness, but it is far more complicated than just a low serotonin level.
However like ancient myths the serotonin myth serves some important purposes.
Explanation. Myths explain things that need explaining. Most of the explanations of ancient mythology have been superseded by scientific explanations so a modern myth has to be based on a scientific model to have any credibility. Being only partially factual does not matter to a myth.
Morality. Myths provide moral guidance. People with depression often feel guilt and feel that they have done something to deserve their situation. They are often blamed by others for not "snapping out of it". Unfortunately there is a stigma attached to mental illness. Presenting it as a chemical deficiency (like diabetes) excludes blame guilt and morality, which is a good thing.
Action. Myths provide guidance in decision making. The serotonin myth helps guide people to make a sensible decision which is to get treatment for their depression. Drugs which boost serotonin are lifesaving and even cognitive behavioral based therapies sometimes get explained in terms of the fact that these therapies will boost serotonin levels and thus help resolve the depression.
For many people ancient myths are now regarded as "wrong" because the explanations they provide have been replaced by better explanations. But such a judgement ignores the valuable role these stories served in their own time. Similarly I have no doubt that one day the serotonin explanation will be regarded as wrong. However that does not matter to us now as the serotonIn story still has much to offer in the treatment of depression.