IF YOU'VE FOUND yourself struggling with persistent melancholy or malaise in recent years, you're not alone: Twenty-one million adults in the U.S. - or 8 percent of the population - had at least one major depressive episode in 2020. That prevalence is even higher, 17 percent of the population, among 18-to 25-year-olds. With so many people affected, it's surprising how little we know about depression in the brain and body. The neurotransmitter serotonin has long been a prime suspect for the condition's cause. That's largely based on the notion that selective serotonin reuptake inhibitor drugs (SSRIs), which increase serotonin levels in the brain, seem to alleviate depression symptoms for many people. But this July, a team led by researchers at University College London did a systematic review of 17 recent studies that looked for a link between serotonin and depression. They didn't find one.
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