IT IS mid-afternoon in Camberwell Green and the fast-food joints are filling up with schoolchildren. Some are in groups of friends, others are picking up dinner with their family. It is hard to escape unhealthy food in this south London neighbourhood. Posters in supermarkets and on bus-stops display deals for fizzy drinks and burgers. Newsagents’ doorways glitter with packets of sweets and crisps. Even a church, converted from a bingo hall, has a snack-dispensing vending machine in its lobby.

The children of Camberwell Green are among the fattest in England. Half of ten- and 11-year-olds there are overweight or obese (meaning that a boy of average height would weigh over 40kg, as opposed to a healthy 35kg or so). By contrast, in Dulwich Village, a few miles south, where household incomes are twice as high, only a fifth of children are in that category, one of the lowest levels in the country.

Poor children have been fatter than rich ones since around the 1980s. But over the past decade the rich have started to slim down, as the poor have got bigger. This is true in poor Camberwell and posh Dulwich, where rates of childhood obesity have respectively risen by ten percentage points and fallen by two in the past six years. And it is true of the country at large (see chart).

Since the 1990s the public has become more aware of the risks of obesity. The rich and well educated are best placed to act on this knowledge, says Ronny Cheung, a paediatrician in London. They have more time to cook healthy meals at home and are more likely than poor folk to live near green spaces or join sports clubs.

At the same time there has been an uneven growth of fast-food joints, explains Thomas Burgoine of the Centre for Diet and Activity Research (CEDAR) at Cambridge University. He and colleagues looked at the density of fast-food outlets in Norfolk between 1990 and 2008. In the poorest areas it increased by two restaurants per 10,000 residents, whereas in the richest it grew by just 0.5. Across England, there are 8.2 fast-food outlets per 10,000 people, and 11.6 in the poorest parts of the country, according to Public Health England, a government agency. The temptation to indulge is furthered by the marketing of fatty foods. Cancer Research UK, a charity, suggests that poor children are more likely than rich ones to be exposed to junk-food ads. People have a limited amount of mental capacity to think about their problems, argues Hugo Harper of the Behavioural Insights Team, a part-publicly owned think-tank which co-authored a recent report on the subject with Guy’s and St Thomas’ Charity. Parents concerned about paying rent and keeping the electricity on are thus less likely to think about cooking a healthy dinner. One experiment found that subjects who were asked to memorise a long string of numbers were more likely than others to pick a chocolate cake over a fruit salad. Stress has a similar effect. And it may be getting costlier to eat well. CEDAR found that in 2002-12 the most nutritious foods, like vegetables and unprocessed meats, were pricier than unhealthy meals such as pizzas and burgers—and that the price gap grew each year by about 10p ($0.13) per 1,000 calories.

Politicians have taken note. In April the government introduced a tax on sugary soft drinks. It is expected to announce a new anti-obesity strategy within weeks; Scotland will set out its own plan in the summer. On May 30th a report by the health select committee of MPs recommended that junk-food ads be banned from television before 9pm and that cartoon characters not be permitted to advertise snacks, among other measures.

But the underlying causes of childhood obesity are fiendishly tricky to fix. As Sir Michael Marmot, head of University College London’s Institute of Health Equity, puts it: “If you want to solve the obesity problem, you have to solve the inequality problem first.”