But Anderson's new research challenges that idea, suggesting instead that suppressing negative thoughts may in fact improve symptoms of anxiety, depression and post-traumatic stress disorder.

In a study published Wednesday in the journal Science Advances, Anderson and his co-author found that training the brain to block out negative thoughts appeared to improve mental health outcomes.

Their research involved 120 adults from 16 countries, who were each asked to list 20 fears about things that might happen in the future, 20 hopes and 36 neutral events, such a visit to the eye doctor.

"The fears couldn’t be generic, like 'I’m worried that aliens are going to land on Earth.' They’re things that are going through your mind recurrently that cause distress," Anderson said.

Next, the participants came up with a word that reminded them of each type of event. For instance, if a person's fear was that their parents would get severely sick with Covid, the word might be "hospital."

Half of the participants were told to stare at one of their negative words for a few seconds without letting their minds wander into more distressing thoughts. For comparison, the other half were given the same assignment, only with their neutral words.

"You’re told: If something does pop into mind, even briefly, push it out," Anderson said. "Moreover, don’t distract yourself. Don’t think about lunch."

The exercise was repeated 12 times per day for three days. At the end of the experiment, participants who blocked out negative thoughts reported that those fears were less vivid and their mental health had improved compared to the group tasked with suppressing neutral thoughts. The results held true three months after the experiment ended.

Participants who reported high levels of anxiety to start saw their self-reported worries decline by 44%, on average. Among people with post-traumatic stress, their overall negative mental health (measured as a combination of self-reported anxiety, depression and worry) fell by an average of 16%, while their positive mental health increased by nearly 10%.

"The people with the highest trait anxiety and the highest PTSD were the ones that benefited the most," Anderson said. "There were no instances, actually, of increases in negative symptoms as a result of this intervention."

What’s more, suppressing negative thoughts seemed to lower the chances that participants’ mental health issues got worse over time.

Three months after the experiment was over, around 80% of participants said they had continued to use the thought suppression techniques they learned in the study to control their fears. Anderson said the researchers looked for evidence that people’s fears were bouncing back or becoming more intense but didn’t find any sign of that.

He thinks training the brain to block out negative thoughts may be an important tool for treating anxiety, depression and PTSD, both in therapy offices and at home.

"Once you teach people what they need to do, I think that they can do it on their own," he said.

However, Jan Wessel, an associate professor of psychological and brain sciences at the University of Iowa, said it’s too soon to recommend such an approach.

"I don't think any individual study of a scale like this should lead to an immediate clinical application," he said. "But I think it's very, very encouraging."

Talking about fears and past traumas can still have benefits

Sigmund Freud, considered the founder of psychoanalysis, introduced the idea that people should talk through their negative thoughts rather than suppressing them.

"Freud said repression is a defense mechanism. It makes you feel better in the moment, but you’re just shoving stuff down into the unconscious and it’s going to come back and influence you indirectly," Anderson said.

Then starting in the 1980s, Harvard social psychologist Daniel Wegner popularized the theory that trying to avoid a thought often backfires by making that thought more pervasive.

But Wessel said those theories aren't well supported by rigorous scientific studies.

"From a basic science perspective, some of these assumptions of psychoanalysis have probably not stood the test of time to the degree that you would want," he said.

Instead, Wessel said, there's evidence that people can train their minds to shut off certain harmful thinking patterns.

"Think about a baseball player learning how to check their swings better. They can really hone their perceptual and cognitive system to get really, really good at stopping themselves from executing an action. We believe that similar mechanisms are probably involved in in the suppression of things like intrusive thoughts," he said.

However, Wessel also noted that some people benefit from the opposite approach: In controlled settings, exposing people with phobias or PTSD to the object or activity they fear can help reduce that fear, research suggests.

Anderson said his new study should not be taken to mean that nobody benefits from talking about their negative thoughts.

"I certainly don’t think that there’s anything wrong with processing significant things in our lives," he said. "But for the majority of negative thoughts that we have, I’m not sure that recipe fits the bill."