In scientific terms, obesity occurs when a person consumes more calories than he or she burns. What causes this imbalance between calories in and calories out may differ from one person to another. Genetic, environmental, psychological, and other factors may all play a part.

Genetic factors

Obesity tends to run in families, suggesting a genetic cause. Yet families also share diet and lifestyle habits that may contribute to obesity. Separating these from genetic factors is often difficult. Even so, science shows that heredity is linked to obesity.

In one study, adults who were adopted as children were found to have weights closer to their biological parents than to their adoptive parents. In this case, the person's genetic makeup had more influence on the development of obesity than the environment in the adoptive family home.

Environmental factors

Genes do not destine people to a lifetime of obesity, however. Environment also strongly influences obesity. This includes lifestyle behaviors such as what a person eats and his or her level of physical activity. Americans tend to eat high-fat foods, and put taste and convenience ahead of nutrition. Also, most Americans do not get enough physical activity.

Although you cannot change your genetic makeup, you can change your eating habits and levels of activity. Try these techniques that have helped some people lose weight and keep it off:

Learn how to choose more nutritious meals that are lower in fat.

Learn to recognize and control environmental cues (like inviting smells) that make you want to eat when you're not hungry.

Become more physically active.

Psychological factors

Psychological factors may also influence eating habits. Many people eat in response to negative emotions such as boredom, sadness, or anger.

Most overweight people have no more psychological problems than people of average weight. Still, up to 10 percent of people who are mildly obese and try to lose weight on their own or through commercial weight loss programs have binge eating disorder. This disorder is even more common in people who are severely obese.

During a binge eating episode, people eat large amounts of food and feel that they cannot control how much they are eating. Those with the most severe binge eating problems are also likely to have symptoms of depression and low self-esteem. These people may have more difficulty losing weight and keeping it off than people without binge eating problems.

If you are upset by binge eating behavior and think you might have binge eating disorder, seek help from a health professional such as a psychiatrist, psychologist, or clinical social worker.

Other causes of obesity

Some illnesses can lead to obesity or a tendency to gain weight. These include hypothyroidism, Cushing's syndrome, depression, and certain neurological problems that can lead to overeating. Also, drugs such as steroids and some antidepressants may cause weight gain. A doctor can tell whether there are underlying medical conditions that are causing weight gain or making weight loss difficult.

Source: U.S. National Institute of Diabetes & Digestive & Kidney Diseases of the National Institutes of Health