Two major groups of factors, genetic and environmental, have a balance that variably intertwines in the development of obesity. Genetic factors are presumed to explain 40-70% of the variance in obesity, within a limited range of BMI (18-30 kg/m2).

A study in which monozygotic twins were overfed by 1000 kcal per day, 6 days a week, over a 100-day period found that the amount of weight gain varied significantly between pairs (4.3 to 13.3 kg). However, the similarity within each pair was significant with respect to body weight, percentage of fat, fat mass, and estimated subcutaneous fat, with about 3 times more variance among pairs than within them. This observation indicates that genetic factors are significantly involved and may govern the tendency to store energy.