The New World has lost its superiority over the Old - at least in terms of physical stature - and John Komlos has the evidence to prove it. His records, including files on "runaway slaves", "indentured servants" and "West Point graduates", bear testament to the American decline.

Prof Komlos's research over more than 20 years has documented the heights of almost a quarter of a million people from the 1700s to today. The findings, he says, provide the most accurate gauge to date with which to measure the development of the human physique.

"Americans have stopped growing while Europeans are increasing in height at quite a pace," says Prof Komlos, 50, a leading "anthropometric historian" who studies such development. He spreads out files on a table to prove his point.

The heights of soldiers who fought in the Crimean War, of American slaves, of present-day Norwegians and the poor of 18th century London are among data he has gleaned from libraries, military academies and passport offices, and assiduously plotted on to detailed graphs.

He has discovered, for instance, that American men were about three inches taller than the Dutch in the 1800s. Now, the tables have turned, and the Dutch - the tallest people in Europe, with an average height of around 6ft 1in - stand three inches above Americans.

While the average American man was, at 5ft 9in, two inches taller than the average Briton at the time of the American War of Independence in 1775, nowadays the former is about half an inch shorter than the latter.

The slide, says Prof Komlos, an American who works in the department of economic history at the University of Munich, dates back to the mid-20th century, and the reasons, he argues, are probably socio-economic.

"From being the tallest in the world for 200 years with the highest per capita income this suddenly stopped," he says. "By the 1950s, the welfare state was already well-established in many European countries. It is an achievement that cannot simply be ignored."

The Dutch, he says, won their anthropometric advantage by creating the world's best pre- and post-natal clinics. Over the same period, America's rich-poor divide has been widening.

Eight million Americans are now unemployed, while 40 million have no health insurance. Infant mortality is twice as high as in Scandinavia, while an increasing reliance on fast food in America means that even the better-off are tending to expand outwards rather than upwards.

Prof Komlos has applied his research to his own family history. He believes that his modest standing of 5ft 7in results from his birth to Hungarian Jewish parents, who fled Budapest in 1944 when he was still in the womb, and could provide him with a only poor diet of bread and broth for the first years of his life.

His direct experience of how height can reflect social conditions has led him to pay particular attention to the poor of previous centuries. "The heights of the Oliver Twists of London cannot be compared with any other heights apart from, say, indigenous populations in the backwoods of Guatemala," he says.

The average lower-class 16-year-old London boy in the 18th century was only 4ft 8in, according to data collected from London's Marine Society which used to prepare the poor for careers at sea.

Even the slaves in America were an average of four inches taller than London's poor, while a Sandhurst graduate was a full seven inches taller than a lower-class Londoner of the same age.

A similar divide existed in France at the time of the 1789 revolution, when the height gap between an ordinary working French man and a graduate of the elite École Polytechnique was 2.7 inches.

"This helps to explain the social tensions of the time," says Prof Komlos. There are those who dismiss anthropometric historians' work as a waste of time. But Prof Komlos retorts: "Height is important. In general, the taller people are, the healthier they are and the happier they are.

"Statistically speaking, tall people are more likely to go on dates, to earn more (about $800 more per inch per year), to live longer - in short, to be more successful. The only things some of them have to worry about are back problems."