Genetics
Childhood obesity is often the result of an interplay between many genetic and environmental factors. Polymorphisms in various genes controlling appetite and metabolism predispose individuals to obesity when sufficient calories are present. Over 200 genes affect weight by determining activity level, food preferences, body type, and metabolism. Having two copies of the allele called FTO increases the likelihood of both obesity and diabetes.As such, obesity is a major feature of a number of rare genetic conditions that often present in childhood:

Prader–Willi syndrome with an incidence between 1 in 12,000 and 1 in 15,000 live births is characterized by hyperphagia and food preoccupations which leads to rapid weight gain in those affected. Bardet–Biedl syndrome
MOMO syndrome
Leptin receptor mutations
Congenital leptin deficiency
Melanocortin receptor mutationsIn children with early-onset severe obesity (defined by an onset before ten years of age and body mass index over three standard deviations above normal), 7% harbor a single locus mutation.One study found that 80% of the offspring of two obese parents were obese in contrast to less than 10% of the offspring of two parents who were of normal weight. The percentage of obesity that can be attributed to genetics varies from 6% to 85% depending on the population examined. Family practices
In the recent decades, family practices have significantly changed, and several of these practices greatly contribute to childhood obesity:
With a decreasing number of mothers who breast-feed, more infants become obese children as they grow up and are reared on infant formula instead.
Researchers also did a cohort study on 19,397 babies, from their birth until age seven and discovered that high weight babies at four months were 1.38 times more likely to be overweight at seven years old compared to normal weight babies. High weight babies at the age of one were 1.17 times more likely to be overweight at age seven compared to normal weight babies. Medical illness
Cushings syndrome (a condition in which the body contains excess amounts of cortisol) may also influence childhood obesity. Researchers analyzed two isoforms (proteins that have the same purpose as other proteins, but are programmed by different genes) in the cells of 16 adults undergoing abdominal surgery. They discovered that one type of isoform created oxo-reductase activity (the alteration of cortisone to cortisol) and this activity increased 127.5 pmol mg sup when the other type of isoform was treated with cortisol and insulin. The activity of the cortisol and insulin can possibly activate Cushings syndrome.Hypothyroidism is a hormonal cause of obesity, but it does not significantly affect obese people who have it more than obese people who do not have it. In a comparison of 108 obese patients with hypothyroidism to 131 obese patients without hypothyroidism, researchers discovered that those with hypothyroidism had only 0.077 points more on the caloric intake scale than did those without hypothyroidism. Psychological factors
Researchers surveyed 1,520 children, ages 9–10, with a four-year follow up and discovered a positive correlation between obesity and low self-esteem in the four-year follow up.