In their book, Obesity in Childhood and Adolescence,  H. Dele Davies, Hiram E. Fitzgerald, and Vasiliki Mousouli discuss some of the causes of childhood obesity.  They propose that environmental and genetic factors play a large role in the issue of childhood obesity. They discuss how the increase of portion size over the last 40 years has led portions to be of an excessive size that now need to be reduced.  The consequence of these larger portion sizes is that they are leading to a higher caloric intake.  In order to support their claims the authors provide significant data from a variety of sources.