We've long blamed television for turning us into a nation of couch potatoes but now the dismal science is weighing in with the idea that it is technology, rather than a change in tastes or the growth of fast food restaurants, that is the cause of obesity.

Professor Carol Propper, of Bristol University, says economics can bring valuable insights into the world of over-eating and says technology has both reduced the cost of food as well as making work and life less strenuous.

She delivered a paper on Friday to the Royal Economic Society that contained many well-known but still shocking figures. Obesity has risen dramatically in the past 10 years alone. More than 21% of men and women in Britain are too heavy for their height. A decade ago the figures were 13% of men and 16% of women. The only country in a worse situation than Britain is the United States.

Obesity causes 30,000 deaths a year and costs the NHS about £500m a year and the economy £2bn through sickness and early death.

So far, so well known. But Prof Propper says it is quite wrong to blame fast-food outlets or convenience food in supermarkets for the problem. They are a reflection of it rather than a cause.

She breaks down her analysis to what has happened to the price of consuming calories and what has happened to cost of expending them.

"We eat more because of improved technology," she says. Improvements in agricultural technology have reduced the price of food while microwave ovens, food preservatives and packaging technology have cut the time to prepare food.

In 1965, non-working married women spent more than two hours a day cooking and cleaning up after meals. Now the equivalent time is less than an hour. This, says Prof Propper, has led to an increase in the quantity and variety of food consumed.

The decrease in the price of food has benefited the poor most and it is the poor who have seen their weight increase the fastest, says Prof Propper.

Calorifically speaking, the cheapest foods are packed with sugar, fat and refined grains. Their long shelf life means they are available in most convenience stores, more often used by the poor. In the US, for example, one in four people below the poverty line are obese while only one in six of the better off are.

Thus, she says, the idea of taxing fast or convenience foods, which is increasingly discussed in Britain and the US, would hit the poor disproportionately hard.

Her study has also found that the main reason for an increase in calorie intake is down to an increase in snacking, particularly in the home, rather than to bigger portions or fattening meals in fast-food restaurants.

Technology has also allowed consumption of chips to rise dramatically. Decades ago lots of potatoes were consumed but generally in boiled, mashed or baked form because chips were too much hassle to prepare. Now chips are mass-produced in factories, frozen and sold to be reheated in microwaves.

Evidence suggests that 40% of weight gain in Britain or the US is due to the effects of better agricultural technology pushing prices down while 60% is due to the so-called "demand effects" of declining physical activity in the home and workplace.

"In a post-industrial society, work entails relatively little exercise. Payment is mostly in terms of forgone leisure, because leisure-based exercise must be substituted for exercise on the job.

"So the cost of expending calories has increased. Together this means weight has risen," says Prof Propper.

Interestingly, to return to couch potatoes, television cannot be blamed for the rise in obesity in the last 20 years. Evidence from both the UK and the US shows that the increase in hours spent watching television rose until 1980 but has not risen since.

Nor can we blame the car. The big rise in people using their car to drive to work also occurred prior to 1980, according to data from the US.