MELBOURNE--

We are getting fatter. In Australia, the United States, and many other countries, it has become commonplace to see people so fat that they waddle rather than walk. The rise in obesity is steepest in the developed world, but it is occurring in middle-income and poor countries, too.

Is a person's weight his or her own business? Should we simply become more accepting of diverse body shapes? I don't think so. Obesity is an ethical issue, because an increase in weight by some imposes costs on others.

I am writing this at an airport. A slight Asian woman has checked in with, I would guess, about 40 kilograms (88 pounds) of suitcases and boxes. She pays extra for exceeding the weight allowance. A man who must weigh at least 40 kilos more than she does, but whose baggage is under the limit, pays nothing. Yet, in terms of the airplane's fuel consumption, it is all the same whether the extra weight is baggage or body fat.

Tony Webber, a former chief economist for the Australian airline Qantas, has pointed out that, since 2000, the average weight of adult passengers on its planes has increased by two kilos. For a large, modern aircraft like the Airbus A380, that means that an extra $472 of fuel has to be burned on a flight from Sydney to London. If the airline flies that route in both directions three times a day, over a year it will spend an additional $1 million for fuel, or, on current margins, about 13 percent of the airline's profit from operating that route.

Webber suggests that airlines set a standard passenger weight, say, 75 kilos. If a passenger weighs 100 kilos, a surcharge would be charged to cover the extra fuel costs. For a passenger who is 25 kilos overweight, the surcharge on a Sydney-London return ticket would be $29. A passenger weighing just 50 kilos would get a discount of the same amount.

Another way to achieve the same objective would be to set a standard weight for passengers and luggage, and then ask people to get on the scales with their luggage. That would have the advantage of avoiding embarrassment for those who do not wish to reveal their weight.