The more disadvantaged groups suffer from higher rates of obesity. This is because at every stage of life the risk factors, known or assumed, for obesity and overweight are amplified in poorer populations. Hence, at the beginning of life the risk is higher for a newborn baby from a family of low socio-economic status of having an obese parent, a mother who smoked during pregnancy, a low birth weight, and of not having been breast-fed. Later, during infancy and adulthood, access to a balanced diet and to opportunities for physical activity is limited by economic and geographical barriers and by psychosocial factors. Lifestyle modifications and eating habits involved in the increase in the prevalence of obesity have had a more unfavorable impact on poorer populations than on the rest of the population, leading to an aggravation of the social obesity gradient. In France, in 1997 there were 3 times as many obese adults in the lowest income decile than in the highest. Today there are 4 times as many. A vicious circle has become established, partly because the causal relations between socioeconomic status and obesity exist in both directions and also because the risk of obesity increases from generation to generation, and, for a given individual, throughout his or her life.