Greenberger and Padesky (1995) explain in their book, Mind Over Mood: Change How You Feel by Changing the Way You Think, that our expectations affect our behavior. People are more likely to try to do something and succeed if they believe it is possible.
For many years, athletes believed that it was absolutely impossible for a human to run a 4-minute mile. In track events all around the world, top athletes ran a mile in just over 4 minutes. Then a man named Roger Bannister, a British miler, decided to determine what changes he could make in his running style and strategy in order for him to break the 4-minute barrier. Bannister believed it was possible to run faster and put many months of effort into altering his running pattern to reach this particular goal. In 1954 Roger 
Bannister became the very first man to run a mile in less than 4 minutes. It was his belief that he could succeed that contributed to the change in the behavior. Remarkably, once Bannister broke the record, the best milers from all around the world also began to run the mile in under 4 minutes. Unlike Bannister, these athletes had not substantially changed their running patterns. The thing that had changed was their thoughts; they now believed it was possible to run this fast and their behavior followed their thoughts.
Just knowing it is possible to run this fast does not mean everyone can do this, of course. Thinking something is not the same thing as doing it. But the more you believe something is possible the more likely you are to attempt it and maybe succeed at it.
Reference: Greenberger, D. & Padesky, C.A. (1995). Mind over mood: Changing how you feel by changing the way you think. New York, NY: The Guilford Press
Cindy Geil, MA
WKPIC Doctoral Intern