Socioeconomic status (SES) (a measure of an individual's or family's social position relative to others) is linked to adiposity,98 with those from lower SES backgrounds in high-income countries found to indulge in more obesogenic behaviours, such as eating more energy-dense food and being less physically active.99 This relationship is found to be stronger for women than men100 and is also true for children,101 with SES in childhood a predictor of obesity in adulthood. However, there is some evidence that women are more likely to adopt the BMI standard of the SES they move to later in life.102 The relationship between SES and obesity can vary by characteristics such as age, sex and ethnicity103,104 with an opposing relationship found in many lower-income countries where obesity can be considered a sign of affluence and lead to less healthy dietary behaviour.105

Some have suggested it is economic inequality which accounts for most of the social inequality in obesity rates,106 with the price elasticity of various foods decreasing with increasing income,107 so that as consumers become more affluent they respond less to price changes in food. The cost of food may thus be one barrier to adopting healthier diets, with studies suggesting that energy-dense and nutrient-poor foods provide dietary energy at lower cost than do more healthy foods.108,109 Additionally less affluent individuals are more concerned by food waste and are therefore less likely to buy foods that will be wasted, leading to more restricted diets.110 Although the economics prove to be important they are not the only factor. There are links between SES and environmental influences, including likelihood of breastfeeding,111–114 nutrition in infancy and childhood, psychological factors and cultural or social norms in relation to diet choice and attitudes to body shape.102 Some studies also report a reverse causality between SES and obesity, in that obesity adversely affects SES. It is possible, however, that external factors may influence both SES and obesity simultaneously.