The "a calorie is a calorie" model of<unk>  posits a combination of excessive food energy intake and a lack of physical activity as the cause of most cases of<unk> . A limited number of cases are due primarily to genetics, medical reasons, or psychiatric illness. In contrast, increasing rates of<unk>  at a societal level are felt to be due to an easily accessible and palatable diet, increased reliance on cars, and mechanized manufacturing. A 2006 review identified ten other possible contributors to the recent increase of<unk> : (1) insufficient sleep, (2) endocrine disruptors (environmental pollutants that interfere with lipid metabolism), (3) decreased variability in ambient temperature, (4) decreased rates of smoking, because smoking suppresses appetite, (5) increased use of medications that can cause weight gain (e.g., atypical antipsychotics), (6) proportional increases in ethnic and age groups that tend to be heavier, (7) pregnancy at a later age (which may cause susceptibility to<unk>  in children), (8) epigenetic risk factors passed on generationally, (9) natural selection for higher BMI, and (10) assortative mating leading to increased concentration of<unk>  risk factors (this would increase the number of<unk> ese people by increasing population variance in weight). According to the Endocrine Society, there is "growing evidence suggesting that<unk>  is a disorder of the energy homeostasis system, rather than simply arising from the passive accumulation of excess weight". Diet alt=(Left) A world map with countries colored to reflect the food energy consumption of their people in 1961. North America, Europe, and Australia have relatively high intake, while Africa and Asia consume much less. 1961 alt=(Right) A world map with countries colored to reflect the food energy consumption of their people in 2001–2003. Consumption in North America, Europe, and Australia has increased with respect to previous levels in 1971. Food consumption has also increased substantially in many parts of Asia. However, food consumption in Africa remains low. 2001–03 Map of dietary energy availability per person per day in 1961 (left) and 2001–2003 (right) Calories per person per day (kilojoules per person per day) Average per capita energy consumption of the world from 1961 to 2002 A 2016 review supported excess appetite for palatable, high-calorie food (especially fat, sugar, and certain animal proteins) as the primary factor driving<unk>  worldwide, likely because of imbalances in neurotransmitters affecting the drive to eat. Dietary energy supply per capita varies markedly between different regions and countries. It has also changed significantly over time. From the early 1970s to the late 1990s the average food energy available per person per day (the amount of food bought) increased in all parts of the world except Eastern Europe. The United States had the highest availability with per person in 1996. This increased further in 2003 to . During the late 1990s, Europeans had per person, in the developing areas of Asia there were per person, and in sub-Saharan Africa people had per person. Total food energy consumption has been found to be related to<unk> . The widespread availability of dietary guidelines has done little to address the problems of overeating and poor dietary choice. From 1971 to 2000,<unk>  rates in the United States increased from 14.5% to 30.9%. During the same period, an increase occurred in the average amount of food energy consumed. For women, the average increase was per day ( in 1971 and in 2004), while for men the average increase was per day ( in 1971 and in 2004). Most of this extra food energy came from an increase in carbohydrate consumption rather than fat consumption. The primary sources of these extra carbohydrates are sweetened beverages, which now account for almost 25 percent of daily food energy in young adults in America, and potato chips. Consumption of sweetened beverages such as soft drinks, fruit drinks, and iced tea is believed to be contributing to the rising rates of<unk>  and to an increased risk of metabolic syndrome and type 2 diabetes. Vitamin D deficiency is related to diseases associated with<unk> . As societies become increasingly reliant on energy-dense, big-portions, and fast-food meals, the association between fast-food consumption and<unk>  becomes more concerning. In the United States, consumption of fast-food meals tripled and food energy intake from these meals quadrupled between 1977 and 1995. Agricultural policy and techniques in the United States and Europe have led to lower food prices. In the United States, subsidization of corn, soy, wheat, and rice through the U.S. farm bill has made the main sources of processed food cheap compared to fruits and vegetables. Calorie count laws and nutrition facts labels attempt to steer people toward making healthier food choices, including awareness of how much food energy is being consumed.<unk> ese people consistently under-report their food consumption as compared to people of normal weight. This is supported both by tests of people carried out in a calorimeter room and by direct<unk> servation. Sedentary lifestyle A sedentary lifestyle plays a significant role in<unk> . Worldwide there has been a large shift towards less physically demanding work, and currently at least 30% of the world's population gets insufficient exercise. This is primarily due to increasing use of mechanized transportation and a greater prevalence of labor-saving technology in the home. In children, there appear to be declines in levels of physical activity (with particularly strong declines in the amount of walking and physical education), likely due to safety concerns, changes in social interaction (such as fewer relationships with neighborhood children), and inadequate urban design (such as too few public spaces for safe physical activity). World trends in active leisure time physical activity are less clear. The World Health Organization indicates people worldwide are taking up less active recreational pursuits, while a study from Finland found an increase and a study from the United States found leisure-time physical activity has not changed significantly. A 2011 review of physical activity in children found that it may not be a significant contributor. In both children and adults, there is an association between television viewing time and the risk of<unk> . A review found 63 of 73 studies (86%) showed an increased rate of childhood<unk>  with increased media exposure, with rates increasing proportionally to time spent watching television. Genetics A 1680 painting by Juan Carreno de Miranda of a girl presumed to have Prader–Willi syndrome Like many other medical conditions,<unk>  is the result of an interplay between genetic and environmental factors. Polymorphisms in various genes controlling appetite and metabolism predispose to<unk>  when sufficient food energy is present. As of 2006, more than 41 of these sites on the human genome have been linked to the development of<unk>  when a favorable environment is present. People with two copies of the FTO gene (fat mass and<unk>  associated gene) have been found on average to weigh 3–4 kg more and have a 1.67-fold greater risk of<unk>  compared with those without the risk allele. The differences in BMI between people that are due to genetics varies depending on the population examined from 6% to 85%.<unk>  is a major feature in several syndromes, such as Prader–Willi syndrome, Bardet–Biedl syndrome, Cohen syndrome, and MOMO syndrome. (The term "non-syndromic<unk> " is sometimes used to exclude these conditions.) In people with early-onset severe<unk>  (defined by an onset before 10 years of age and body mass index over three standard deviations above normal), 7% harbor a single point DNA mutation. Studies that have focused on inheritance patterns rather than on specific genes have found that 80% of the offspring of two<unk> ese parents were also<unk> ese, in contrast to less than 10% of the offspring of two parents who were of normal weight. Different people exposed to the same environment have different risks of<unk>  due to their underlying genetics. The thrifty gene hypothesis postulates that, due to dietary scarcity during human evolution, people are prone to<unk> . Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would be more likely to survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies. This theory has received various criticisms, and other evolutionarily-based theories such as the drifty gene hypothesis and the thrifty phenotype hypothesis have also been proposed. Other illnesses Certain physical and mental illnesses and the pharmaceutical substances used to treat them can increase risk of<unk> . Medical illnesses that increase<unk>  risk include several rare genetic syndromes (listed above) as well as some congenital or acquired conditions: hypothyroidism, Cushing's syndrome, growth hormone deficiency, and some eating disorders such as binge eating disorder and night eating syndrome. However,<unk>  is not regarded as a psychiatric disorder, and therefore is not listed in the DSM-IVR as a psychiatric illness. The risk of overweight and<unk>  is higher in patients with psychiatric disorders than in persons without psychiatric disorders. Certain medications may cause weight gain or changes in body composition; these include
Obesity
Heart failure
Body mass index
Metabolic syndrome
Abdominal obesity
Family history 
Dry sex
Smoking