See also:
The Serotonin Theory of Depression: a Systematic Umbrella Review of the Evidence (2022). It's worth reading the introduction and results in full, but here are two important quotes (footnote markers removed):
> Our comprehensive review of the major strands of research on
serotonin shows there is no convincing evidence that depression
is associated with, or caused by, lower serotonin concentrations or
activity. Most studies found no evidence of reduced serotonin
activity in people with depression compared to people without,
and methods to reduce serotonin availability using tryptophan
depletion do not consistently lower mood in volunteers. High
quality, well-powered genetic studies effectively exclude an
association between genotypes related to the serotonin system
and depression, including a proposed interaction with stress.
> The chemical imbalance theory of depression is still put forward
by professionals, and the serotonin theory, in particular, has
formed the basis of a considerable research effort over the last few
decades. The general public widely believes that depression
has been convincingly demonstrated to be the result of serotonin
or other chemical abnormalities, and this belief shapes
how people understand their moods, leading to a pessimistic
outlook on the outcome of depression and negative expectancies
about the possibility of self-regulation of mood. The idea
that depression is the result of a chemical imbalance also
influences decisions about whether to take or continue anti-depressant medication and may discourage people from dis-continuing treatment, potentially leading to lifelong dependence
on these drugs.
https://www.nature.com/articles/s41380-022-01661-0
> Our comprehensive review of the major strands of research on serotonin shows there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity. Most studies found no evidence of reduced serotonin activity in people with depression compared to people without, and methods to reduce serotonin availability using tryptophan depletion do not consistently lower mood in volunteers. High quality, well-powered genetic studies effectively exclude an association between genotypes related to the serotonin system and depression, including a proposed interaction with stress.
> The chemical imbalance theory of depression is still put forward by professionals, and the serotonin theory, in particular, has formed the basis of a considerable research effort over the last few decades. The general public widely believes that depression has been convincingly demonstrated to be the result of serotonin or other chemical abnormalities, and this belief shapes how people understand their moods, leading to a pessimistic outlook on the outcome of depression and negative expectancies about the possibility of self-regulation of mood. The idea that depression is the result of a chemical imbalance also influences decisions about whether to take or continue anti-depressant medication and may discourage people from dis-continuing treatment, potentially leading to lifelong dependence on these drugs.
https://www.nature.com/articles/s41380-022-01661-0