In this classic experiment, participants in two groups were told either not to think of a white bear, or that they could if they wanted to. The findings-that trying not to think about something makes you think about it more-can explain some of the struggles experienced by people with OCD.
Perhaps most interesting though, was what happened when subjects in the suppression condition subsequently performed another stream of consciousness session, however this time were told that they were allowed to think of a white bear. Now, these participants who originally attempted to suppress the white bear thought spent an overwhelming amount of time discussing white bears and pressing the button, significantly more so than the group that was allowed to think of a white bear without any previous suppression. It appeared that suppression of a thought led to its subsequent overindulgence."