Why Americans just keep on getting fatter is a question that has obsessed health professionals, sociologists and politicians for decades.

David Cutler, Edward Glaeser and Jesse Shapiro from the Institute of Economic Research at Harvard University, have mined almost a century's-worth of nutritional data in a hunt for lessons.

According to their survey, the real culprit is not sluggish lifestyles or even fatty food, but technology.

As technological advances have made food ever more varied and convenient, the authors argue, the feeble will-power of the American public has been unable to compete.

Big problem

Almost all the rich world is struggling with bulging waistbands, but America is something special.

Increased calorie consumption can, of course, be a function of increasing wealth, and the average American swelled during the 20th century as result.

The average calorie intake rose by about 10 % between the mid 1970s and the mid 1990s.

Now, according to some calculations, close to one-third of Americans are clinically obese - about 50% more than even the chubbiest equivalent country.

Old ideas

It finds no evidence for one popular argument - that American portions are getting bigger.