In Joshua Medcalf's mini book, he begins by explaining that strong beliefs cause the Reticular Activating System to block out information that contradicts those beliefs. Medcalf references a Harvard Study that showed that "beliefs are so powerful they can change the chemical properties of drugs." In this study 50% of the participants experienced the effects of a stimulant/barbiturate even though they were given the opposite.
This relates to youth athletes and their mental strength before, during, and after competition. In other words, if a youth athlete believes he/she will be successful there is a greater chance of that child being successful. It also means that if a youth athlete has a negative outlook on any aspect of the sport it will be difficult to change this child's mind.