The answer, after methodical study of brasseries, eateries, pizza parlours, Chinese restaurants and Hard Rock cafes in both countries, is simple: the French eat less of everything. And they eat less because they are served smaller portions.

The French paradox has baffled European and US scientists for more than a decade.

Only 7% of the French are obese, compared with a whopping 22% of all Americans.

Coronary heart disease is the biggest killer in the US, but not in France. Yet the French smoke Gitanes, breakfast on buttery brioche, lunch and dine off confit of duck, sausage, fat goose livers and camembert. They drink wine, round off their meals with cognac, and while away the afternoon with strong coffee and mouthwatering pastries.

Researchers have offered a confusing range of speculation over the years. There might be something specially healthy about goose fat, one group argued. Alcohol, olive oil and salad were all important in a heart-smart diet, said another. Red wine itself might explain the difference, said a third.

So scientists from Philadelphia and the French research agency CRNS in Paris teamed up to compare servings in comparable dining places in the two cities.

They report in the September issue of Psychological Science that they combed restaurant guides, sampled soft drinks and other purchases in supermarkets, and sat down at 11 pairs of eateries to weigh the portions dished up.

Mean portion size across all Paris establishments was 277g (9.8oz), compared with an average in Philadelphia of 346g (12.2oz) - about 25% more. Only in the Hard Rock Cafe chain did the Parisian portions match the US ones.

Philadelphia's Chinese restaurants served 72% more than the Parisian ones. A supermarket soft drink in the US was 52% larger, a hotdog 63% larger, a carton of yoghurt 82% larger.

The lesson is that though the French diet was rich in fat, overall, the Americans consumed more calories. Over the years, this would lead to substantial differences in weight.

"If food is moderately palatable, people tend to consume what is put in front of them, and generally consume more when offered more food," said Paul Rozin, a psychologist at the University of Pennsylvania. "Much discussion of the obesity epidemic in the US has focused on personal willpower, but our study shows that the environment also plays an important role, and that people may be satisfied even if served less than they would normally eat."