What causes this imbalance between consuming and burning calories is unclear.
Evidence suggests that obesity often has more than one cause.
Genetic, environmental, psychological, and other factors all may play a part. However, family members share not only genes but also diet and lifestyle habits that may contribute to obesity. Separating these lifestyle factors from genetic ones is often difficult. Still, growing evidence points to heredity as a strong determining factor of obesity. In one study of adults who were adopted as children, researchers found that the subjects' adult weights were closer to their biological parents' weights than their adoptive parents'.