It's a particularly vexing variant of Murphy's Law: The very thing that you don't want to think about--money worries when you're trying to sleep, a cigarette when you're trying to quit--persistently pops into your head. Psychologist Daniel Wegner, Ph.D., says that the effort to restrict thought is itself to blame: "Trying to control your mind can produce the very state you are trying to avoid."
Such perversity may be the product of what Wegner, a professor at the University of Virginia, calls the theory of ironic mental control processes. While one part of our brain searches for a distraction from unwanted cognitions, another checks to make sure that the taboo thought isn't intruding. Usually, the first system--called the "intentional operating process"--works in tandem with the second, dubbed the "ironic monitor." During times of stress, however, the operating process may be overpowered by its complement, bringing all the things we don't want to think about into consciousness.