Causes

At an individual level, a combination of excessive caloric intake, lack of physical activity, and genetic susceptibility is thought to explain most cases of obesity, with a limited number of cases due solely to genetics, medical reasons, or psychiatric illness.[72] In contrast at a societal level increasing rates of obesity are felt to be due to an easily accessible and palatable diet,[73] increased reliance on cars, and mechanized manufacturing.[74][75]
A 2006 review identified ten other possible contributors to the recent increase of obesity: (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 obesity in children),

(8) epigenetic risk factors passed on generationally,

(9) natural selection for higher BMI, and

(10) assortative mating leading to increased concentration of obesity risk factors (this would not necessarily increase the number of obese people, but would increase the average population weight).

There is substantial but not conclusive evidence for these mechanisms, and the authors specify that they are probably less influential than the ones discussed in the previous paragraph (but still important).