If the eyes have a tendency to deviate outward, the child must use excess effort and energy to maintain fixation on the reading task. Most studies have shown that the greater the amount of effort involved in reading, the lower will be the comprehension and the lower will be the performance. When reading, the eyes do not move smoothly over a line of print. Rather, they make a series of fixations looking from word to word. When an exophoria exists, each time fixation is broken and moved to the next word, the eyes will tend to deviate outwards and they must be brought back in to regain fixation.
Human nature being what it is, the child generally has an avoidance reaction to the reading task.
This is compounded by the fact that anything the child doesn't do well, he would rather not do. This is the child who looks out the window rather than paying visual attention. He is commonly given labels. He is often accused of having a short attention span and not trying. He is told that he would do better if he tried harder, but he has tried harder to no avail.