This is from the Surgeon General:
THE CAUSES OF OVERWEIGHT
Overweight in children and adolescents is generally caused by lack of physical activity, unhealthy eating patterns, or a combination of the two, with genetics and lifestyle both playing important roles in determining a child's weight.
Our society has become very sedentary. Television, computer and video games contribute to children's inactive lifestyles.
43% of adolescents watch more than 2 hours of television each day.
Children, especially girls, become less active as they move through adolescence.