OCD and Serotonin: When The Brain Fails To Realize That The Danger Has Passed and Gets Trapped in Repetitive Thoughts

Serotonin may be doing much more than scientists imagined. A new study has found that it helps the brain "let go" of old thoughts, something that may explain why people with obsessive-compulsive disorder get stuck in repetitive doubts and obsessions. The discovery could transform how the disorder is treated.

Obsessive-compulsive disorder is a mental condition marked by repetitive, intrusive, and difficult-to-control thoughts, called obsessions, as well as compulsive behaviors that the person feels the need to repeat.

For many years, scientists believed that these compulsions functioned mainly as "automatic habits," almost like an autopilot for the brain. However, a new study has brought a much deeper and more surprising explanation: perhaps the real problem lies in the brain's difficulty in perceiving that reality has changed.

Researchers discovered that serotonin, a chemical substance important for communication between neurons, seems to help the brain abandon old beliefs when new evidence emerges. In people with obsessive-compulsive disorder, this process may fail.

This means that even when everything indicates that there is no more danger, doubt, or contamination, the brain remains "stuck" in the previous belief. It's as if the mind has difficulty updating its conclusions about the world around it.

To investigate this, scientists conducted a carefully controlled clinical study. Fifty healthy volunteers participated in an experiment in which half received a placebo and the other half received a single dose of escitalopram, an antidepressant widely used in the treatment of obsessive-compulsive disorder and depression.

Neither the participants nor the researchers knew who had received the real medication during the experiment, preventing expectations from influencing the results.

Afterwards, the volunteers participated in a computer game specifically designed to test how the brain reacts to unexpected changes. In the game, participants had to collect virtual shells. Some contained pearls and awarded points. Others contained dirt and caused the person to lose points.

The important detail is that, throughout the game, the rules changed silently. Without warning, the "good" shells became bad and vice versa. To perform well, players needed to quickly perceive that the environment had changed and adapt their choices.

This method allowed scientists to observe something very interesting: some people continued to insist on old choices even when the evidence clearly showed that they no longer worked. The researchers called this "persistence of belief," a tendency of the brain to remain stuck in an old interpretation of reality.

Participants with higher levels of escitalopram in their blood were able to perceive the changes more quickly and adapt their behavior better. In other words, serotonin seems to have helped the brain "detach" from old beliefs and update its perception of the environment.

Researchers believe this may explain many symptoms of obsessive-compulsive disorder. For example, a person may wash their hands repeatedly because their brain cannot fully register that they are already clean.

Even seeing that there is no dirt, the feeling of threat remains. The problem, therefore, may not be just an excessive habit, but a deeper flaw in the brain's ability to update information and abandon old mental states.

Another important point of the study is that it could change how treatments are carried out. Scientists suggest that antidepressants that increase serotonin may create a temporary “window of flexibility” in the brain.

During this period, the person might respond better to psychotherapy, especially therapies that help confront fears and correct obsessive thoughts. This opens the possibility of more strategic treatments, combining medication and therapy sessions precisely at the times when the brain is most prepared to change.

Although more research is still needed, the study offers a new way to understand obsessive-compulsive disorder. Instead of only looking at repetitive behaviors, scientists are now investigating how the brain deals with doubts, changes, and updating beliefs. This discovery could represent an important step towards more effective treatments and a more human understanding of how the mind of someone trapped in obsessive thoughts works.

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Serotonin reduces belief stickiness

Vasco A. Conceição, Frederike H. Petzschner, David M. Cole, Katharina V. Wellstein, Daniel Müller, Sudhir Raman, and Tiago V. Maia

Nature Mental Health. 4, pages 775-791 (2026) 

DOI:10.1038/s44220-026-00621-9

Abstract: 

Serotonin fosters cognitive flexibility, but how, exactly, remains unclear. We developed a computational theory that proposes that serotonin reduces belief stickiness: the tendency to get ‘stuck’ in a belief about the state of the world despite incoming contradicting evidence. We tested this theory in a randomized, double-blind, placebo-controlled study using a single dose of the selective serotonin reuptake inhibitor escitalopram. In the escitalopram group, higher escitalopram plasma levels reduced belief stickiness more, resulting in better inference about the state of the world. Moreover, participants with sufficiently high escitalopram plasma levels had less belief stickiness, and therefore better state inference, than participants on placebo. We also propose that obsessions may result from excessive belief stickiness. Indeed, participants with more obsessions had greater belief stickiness, and therefore worse state inference. The opposite relations of escitalopram and obsessions with belief stickiness may explain the therapeutic effect of selective serotonin reuptake inhibitors in obsessive–compulsive disorder.