Have you ever found yourself about to give a speech or sit an exam, and one of your friends tries to calm you down by saying, “Stop worrying … just don’t think about it!” Does that ever work? Not usually! The more you try to intentionally block it from your mind, the more it wants to pop up again.
Why is that? It seems intuitive that if you don’t want to think about something, all you need to do is to take control and block it out of your mind, right?
One of Hollywood’s better movies in recent times was “Inception”. In one of the key early scenes, Arthur is explaining to Saito why inception is impossible,
“Saito: If you can steal an idea, why can’t you plant one there instead?
Arthur: Okay, this is me, planting an idea in your mind. I say: don’t think about elephants. What are you thinking about?
This is a great little dialogue about thought suppression. Thought suppression is the process of consciously trying to avoid certain thoughts, either by trying to replace the unwanted thought with another thought, or simply trying to repress the unwanted thought.
Our minds tend to focus on the content of a subject. If the subject is elephants, no matter what words I put in front of it, your mind will think about elephants. Like if I say, “I love elephants, or I say “Don’t think about elephants”, your brain hears, “blah blah blah elephants.” And having been sensitized to the idea of not thinking about elephants, when your mind inevitably brings it up again, you’re primed to pay even more attention to it, “D’oh, I’ve just thought about elephants again … stop thinking about elephants …”.
This phenomenon is even more pronounced if your mind has already been focusing on the subject. If you’re mind is going over and over a speech you have to give and I say, “Oh, don’t worry about that speech”, all your mind registers is, “blah blah blah SPEECH”.
Although it’s been discussed in the psychological sciences for decades, it’s only been since the late 1980’s that considerable attention has been given to the concept of thought suppression. Despite our natural tendencies to try it or recommend it to people, the conclusion of nearly all the research is the same: thought suppression doesn’t work.
Wenzlaff and Wegner, two American psychology researchers, looked at all of the different research on thought suppression and came to the following conclusion,
“What has compelled the interest of the scientific and clinical communities is that suppression is not simply an ineffective tactic of mental control; it is counterproductive, helping assure the very state of mind one had hoped to avoid. The problem of thought suppression is aggravated by its intuitive appeal and apparent simplicity, which help mask its false promises.” 
I’m not really sure why we naturally gravitate to thought suppression. Perhaps it’s part of our natural delusion of control. Perhaps it’s a throwback from the pop-psychology assumptions that we can control our destiny, or the common myth that our mind is in control of our brain.
Whatever the reason, as time has passed, researchers are coming to understand why thought suppression is so unhelpful. This quote from Magee and his colleagues helps to explain why:
“This shift in focus parallels advances in cognitive theories of intrusive thoughts, which suggest that having intrusive thoughts is a normative phenomenon; instead, the way an individual interprets those thoughts is expected to lead to benign versus serious outcomes … Similarly, having difficulties with thought suppression is a common experience … it is the way an individual interprets that experience that may be key. Previous discussions of thought suppression have frequently implied that people having difficulties with thought suppression often ascribe negative meaning to their difficulties.” 
We naturally struggle to suppress intrusive thoughts because intrusive thoughts are normal. Trying to suppress them is like trying to suppress any other normal biological process. Try to stop breathing for any length of time and you’ll see what I mean – it’s impossible, and trying is simply counterproductive.
The key is how we react to or feel about our thoughts. If we feel like our thoughts might be somehow causing us harm, then our failure to stop them from bubbling up to the surface of our consciousness is going to cause us distress. It’s a double whammy – we’re stressed because we’re expecting the negative consequences of our thoughts, and we’re distressed by our ‘failure’ to stop them.
Since it first started more than a century ago, the death toll from the famous Pamplona event, “Running of the Bulls” currently stands at 13. Countless others have been gored and trampled. Who are the people who get injured during the event? Certainly not the smart ones standing behind the barriers on the edge of the streets, or the ones watching it broadcast on TV? Only the morons who try to outrun the pack of foot-long bony skewers attached to the half-ton lumps of very cranky steak.
Similarly, the best way to manage our thoughts is to learn not to fight with them in the first place. By non-judgmentally observing them, we can simply observe our thoughts for what they are … just thoughts. By stepping back from our thoughts and giving them room, we find that they don’t have any real power over us. Stepping back away from our thoughts and letting them be is the skill of defusion, one part of the process of psychological acceptance. It’s the first step in living a life abundant in meaning and significance.
So just remember: don’t try to suppress an unwelcome thought. Having intrusive thoughts is actually a normal process, not a sign of disease or mental weakness. They’re not toxic or harmful, they’re just thoughts. Give them space, like you would a charging angry bull (or elephant!)
 Wenzlaff RM, Wegner DM. Thought suppression. Annual review of psychology 2000;51(1):59-91.
 Magee JC, Harden KP, Teachman BA. Psychopathology and thought suppression: a quantitative review. Clinical psychology review 2012 Apr;32(3):189-201.