You’re standing on a railway platform, waiting for a train. Everything is perfectly ordinary. Then, without warning, an unsettling thought enters your mind. What if you suddenly jumped onto the tracks?
You have no intention of doing anything of the sort. You weren’t even thinking about it a moment earlier. Yet the thought appeared anyway, as though your brain had briefly decided to go somewhere without your permission.
It’s a strange experience, but far from unusual. Our minds regularly produce thoughts we never consciously chose, including ones that seem completely at odds with what we actually want.
So why does the brain sometimes think the very things we’d rather it didn’t?
In a 2014 international study, researchers interviewed 777 university students across 13 countries on six continents. An astonishing 93.6% reported experiencing at least one unwanted intrusive thought, image or impulse during the previous three months.
The researchers found that these experiences were widespread across cultures. Thoughts involving doubt were particularly common, while more disturbing or taboo subjects were reported less frequently.
Although the study involved university students rather than the general population, what it found suggests that unwanted thoughts are a very ordinary part of the human experience.
The brain is constantly processing information, drawing on memories, emotions and associations, even when we’re not aware and directing our attention towards them.
Sometimes, this mental activity produces an image that surprises us, an idea or a possibility. A passing thought might be related to something we’ve experienced before, something happening around us or a concern we weren’t aware of and considering.
Emotions can also influence which thoughts attract our attention. Anxiety, for example, can make possible dangers seem particularly important, even when there is no immediate threat.
However, scientists do not have a single, definitive explanation for why a particular intrusive thought appears at a particular moment.
In 1985, British psychologist Paul Salkovskis proposed an influential theory about intrusive thoughts and obsessive-compulsive disorder (OCD).
He suggested that an intrusive thought can become particularly distressing when someone sees it as personally significant, especially when they associate it with responsibility, blame or the possibility of harm to themselves or others.
In other words, the thought itself may be less important than the meaning we can attach to it.
Another idea explored by psychologists is thought–action fusion. This describes the notion that merely thinking about an unacceptable action is morally equal to carrying it out, or that imagining an unwanted event somehow makes it more likely to happen.
Research has linked these beliefs to obsessive-compulsive symptoms, which helps explain why certain thoughts can become so difficult to dismiss.
In 1987, psychologist Daniel Wegner and his colleagues conducted an experiment that revealed something unusual about the way we try to control our thoughts.
Participants were asked not to think about a white bear.
Unsurprisingly, they struggled. But the most interesting result came afterwards. When participants were finally allowed to think about the bear, those who had previously attempted to suppress it reported more thoughts about it than participants who had been allowed to think about it from the beginning.
The experiment helped show that there is a paradox: deliberately trying to keep something out of our minds can sometimes make it more harder to forget.
One explanation is that checking ourselves for an unwanted thought requires us to keep observing whether it has returned. In doing so, we may unintentionally keep the thought accessible.
Later research has found evidence of this rebound effect, although it does not occur consistently in every situation.
Thought suppression, it seems, can sometimes achieve precisely the opposite of what we intended.
For most people, unwanted thoughts are fleeting experiences. They appear, attract little attention and eventually disappear.
For others, they can become persistent and deeply distressing.
Obsessive-compulsive disorder involves recurring obsessions, often accompanied by compulsions intended to reduce anxiety or prevent a feared outcome.
According to the NHS, compulsions can involve visible behaviours, such as repeatedly checking that a door is locked, but they can also take place entirely within someone’s mind.
Mental compulsions might include silently repeating phrases, reviewing memories or attempting to neutralise an unwanted thought. Some people repeatedly seek reassurance or avoid situations that trigger their fears.
The informal term Pure-O is sometimes used to describe OCD dominated by intrusive thoughts and less visible compulsions. However, it is not a separate diagnosis, and the term can be misleading because mental compulsions may still be present.
The important distinction is that experiencing an intrusive thought does not, by itself, mean someone has OCD. The disorder involves a persistent pattern that causes a huge amount of distress or disturbs their everyday life.
Established treatments include cognitive behavioural therapy incorporating exposure and response prevention (ERP), and medication such as selective serotonin reuptake inhibitors (SSRIs). The most appropriate approach depends on the person’s circumstances and how severe their symptoms are.
Perhaps the weirdest thing about intrusive thoughts isn’t their content, but the fact that they appear at all. We can control so much of what we do, yet we cannot always control what enters our minds.
An unwanted thought can arrive without warning, contradict everything we believe and disappear just as mysteriously. Sometimes, the harder we try to understand or dismiss it, the more attention we give it. And perhaps that’s the greatest irony of all.
It’s a curious reminder that the human mind doesn’t always behave as we’d expect. Even inside our own heads, we aren’t entirely in charge.
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