top of page
The Problem with the Chemical Imbalance Theory of Mental Illness
By Gillian Hilscher

Anyone familiar with mental illness has almost certainly heard of “serotonin” before. A neurotransmitter often associated with boosting one’s mood, serotonin (or a lack thereof) was long thought to be the culprit behind mental illnesses such as depression. In their 2015 paper, “Antidepressants and the Chemical Imbalance Theory of Depression: A Reflection and Update on the Discourse,” researchers Jeffrey R. Lacasse and Jonathan Leo expose the scientifically unfounded nature of the serotonin theory of depression (known more generally as the chemical imbalance theory), and criticize psychiatry professionals and the pharmaceutical industry for perpetuating this myth for so long.

The chemical imbalance theory of mental illness proposed that mental illnesses were caused by certain deficiencies in key neurotransmitters within the brain. Under this theory, for example, it was assumed that depression was caused by a lack of sufficient serotonin. But by the early 2000s, this theory had largely lost its hold on the scientific community due to an absence of convincing, rigorous evidence in support of it.

Lacasse and Leo explain that a series of clinical trials failed to show a significant difference between a serotonin-boosting medication (called “selective serotonin reuptake inhibitors,” or SSRIs) and a placebo medication. But they also note that patients taking SSRIs did experience improved symptoms. So they—along with psychiatrist Daniel Carlat—suggest that simply starting SSRIs and believing you are on the way to recovering from an illness could alleviate some symptoms. But despite evidence that SSRIs may work, scientists still don’t know enough about the pathophysiology of mental illness for us to be sure of what actually causes it in the first place.

SSRIs may improve symptoms, but the mech

Lacasse and Leo strengthen their argument by including several quotes from other scientific papers, some of which attack the chemical imbalance theory and some of which support it. This support, however, largely stems from a clinical standpoint: it is recommended to adopt the chemical imbalance theory if nothing else than to give patients a satisfying explanation of their condition and put them at ease, even if the theory is not based on rigorous science. Meanwhile, the bulk of the quotes that invalidate the chemical balance theory explain that scientists still don’t know what “normal” chemical balances look like, so to declare that mental illnesses are caused by some kind of imbalance is overstated and irresponsible. The quotes also describe some studies in which certain antidepressants worked by depleting serotonin, rather than increasing it. This all suggests that the assumption that mental illnesses are caused by a neurotransmitter deficiency is naive and overly simplistic. 


Lacasse and Leo are particularly critical of the drug advertising companies that initially pounced on the incomplete mental illness research of the late 20th century, and filled in the blanks by claiming that mental illness medications corrected a pre-existing chemical imbalance. They close out their argument by condemning the new narrative that psychiatric and pharmaceutical professionals are only now learning of the inaccuracy of the chemical imbalance theory; they claim that these industries had been aware of the theory’s problems for years, but continued to perpetuate it perhaps for monetary gain or a desire to please patients (although this is speculation). 

So what do you think? Is it better to give patients an explanation that relieves some of their symptoms, or to be completely transparent about the science behind it?

bottom of page