Overweight Causes.

Genetic Causes
Genetics play a major role in overweight and obesity. Scientific consensus is emerging that body weight is about 70 percent determined by inheritance. Adopted children, for example, tend to resemble the weight of their biological parents more than their adopted parents. Exactly how genes control body weight is not known, but the resting metabolic rate, which is the amount of energy we expend at rest, and the metabolic energy we expend after a meal are about 70 percent determined by our genes. Food and beverage intake, selection of specific food items, and how full we feel after eating may all be at least partially determined genetically.

Nutritional Causes
Americans today are eating more calories than ever before, as indicated by statistics from the Centers for Disease Control and Prevention (CDC):

It’s no mystery why Americans are gaining more weight: we are eating more calories. Specifically, we are eating more carbohydrates, mostly as refined grains, added sugars, and sweeteners; and we are still eating too much of the wrong kinds of fat – animal and trans fats. All of this may lead to premature death.
Another nutritional cause of obesity is that we eat out more often than we used to. About 50 percent of food spending today is spent on food eaten away from home. Eating out contributes to a higher intake of added fats and carbohydrates. Further, the intake of fruits, vegetables and milk is lower when eating out. Seventy-five percent of students ages 11-18 eat in a fast-food restaurant each week.