The most popular idea for what causes depression in the brain is some kind of chemical imbalance. Many pharmaceutical companies have claimed in their advertising that their antidepressant drugs work by correcting this imbalance. The notion has even been popular with some mental health campaigners, who have been keen to show that depression has a 'real' biological basis, rather than just being all in the mind.
However, while many antidepressants do affect the various levels of chemicals in the brain - especially the neurotransmitters serotonin, dopamine, noradrenaline and glutamate - most experts now agree that the idea depression is caused by a chemical imbalance is a gross oversimplification.
Especially as no one knows what the ‘correct’ levels should be. A landmark review of the literature published in 2023 even went so far as to conclude that there’s no “convincing evidence of a biochemical basis to depression”, although that review was controversial, with some commentators arguing that it went too far.