Every now and then, America draws a cartoon of herself for the amusement of the rest of the world. This week's fat lawsuit against McDonald's is one of those occasions.

The New York suit against the fast-food company alleges that a McDonald's store on the Bronx's Bruckner Boulevard is responsible for the size of a 270 lb-child from the neighbourhood and for that of her equally corpulent fellow plaintiffs. The corporation itself is also responsible, by extension, for childhood obesity across America. By failing to label its fries and Big Macs, McDonald's tricks Americans. McDonald's therefore, and not families, is to blame for children's overeating. So now the company must pay huge sums to any number of parties. Or so goes the argument.

The professional cartoonist could not do better. Where else to find three hard-and-fast stereotypes - Americans are enormous porkers, eat fast food to excess and launch crazy lawsuits - united in one picture?

But Pelman v McDonald's is newsworthy beyond its entertainment value. The lawsuit also reveals the consequences of half a century of incremental changes in US common law that have undermined the culture of individual responsibility. They have also established the notion of the civil action as a vehicle of general economic redistribution.

The story starts in 1928, with another cartoon of a lawsuit. Helen Palsgraf, a charwoman, was waiting for a train to Rockaway Beach. A man nearby dropped a packet of fireworks on the station track by accident. A train ran over the packet, causing an explosion. This explosion caused a weighing machine to fall upon Mrs Palsgraf. She sued the railroad, blaming it for this circus-style chain of events.

An appeals court found that the responsibility for Mrs Palsgraf's injuries did not "reach" the railroad. Palsgraf v Long Island Railroad went down as the standard demonstration of the ridiculousness of assigning blame for the risks of life to distant or incidental parties. Courts of the same era determined that makers of products that were outright dangerous - poison, say - bore heavy responsibility for their products. But generally useful consumer products that might be intentionally abused were a different matter. These products were not "unreasonably dangerous" and so demanded fewer safeguards.

Thus in the 1960s, the "Restatement of Torts", a classic of the profession, provided a good example of a product that does not require warning labels: butter. "Good butter is not unreasonably dangerous merely because, if such be the case, it deposits cholesterol in the arteries and leads to heart attacks."

What holds for butter ought to hold for the odd Egg McMuffin - and certainly would have, back in the 1960s. But, in recent decades, plaintiffs have pushed the line of responsibility further and further towards manufacturers and retailers. Judges have refused to dismiss cases blaming companies. And juries have taken to the blame game with a vengeance. Generally speaking, US culture has come to view litigation as a useful substitute for legislation. Indeed, the awards and settlements for these cases, numbering first in the millions and then in the billions, are nowadays viewed as a desirable form of social justice, especially in relation to allegedly addictive products.

In the retail world, the largest example of such jurisprudence is tobacco. (Asbestos is probably the largest overall.) Tobacco's health effects have been officially recognised for four decades now. Yet juries continue to find for plaintiffs who smoked long after they knew of smoking's dangers. Many smokers' cases have also been dismissed or overturned. But the view of the smoker as victim has become more popular. Big tobacco's culpability was recently made official with the states' multi-billion-dollar settlement with tobacco companies.

This background helps explain public receptiveness to the improbable McDonald's suit. Last week, the Good Morning America show invited the mother of a 400 lb male co-plaintiff to appear. She challenged McDonald's with a straight face. Although her son ate three or four McDonald's meals each day, she said: "I had no idea he was destroying himself."

The idea of a boy stuffing himself with "supersized" portions is painful. So is the idea that a mother would believe, or pretend to believe, that this was not her responsibility and be rewarded for doing so.

But rewarding is a goal here. The Big Mac contains only half the calories of the standard serving of fettucine Alfredo; the Quarter Pounder has 70 fewer calories than Starbucks' venti caffe mocha with whipped cream. McDonald's is the target not because of its "supersized" offers but because of its deep pockets.

Still, beyond the money is the issue of the American soul. The McDonald's lawsuit may well be dismissed but more like it will follow. And, in the end, the country will have to decide which cartoon it belongs in, Pelman or Palsgraf.

© Copyright 2002 Financial Times