Height tells you about a segment of the population that is invisible to traditional economic statistics. Children don't have jobs or own houses. They don't buy durable goods, or invest in the stock market. But obviously, investments in their well-being are critical to a nation's economic future.

For several years now, John Komlos, an economic historian at the University of Munich, and other researchers have been trying to figure out exactly why the United States fell behind. How could the wealthiest country in the world, during the most robust economic expansion in its history, simply stop growing?

"It's absolutely fascinating," said Eileen Crimmins, a demographer at the University of Southern California. "Maybe we've reached the point where we're going to go backwards in height."

Like many human traits, an individual's height is determined by a mix of genes and environment. Some experts put the contribution of genes at 40 percent, some at 70 percent, some even higher. But they all agree that aside from African pygmies and a few similar exceptions, most populations have about the same genetic potential for height.

That leaves environment to determine the differences in height between populations around the world, specifically the environment children experience from the moment of conception through adolescence. Any deficiency along the way, from poor prenatal care to early childhood disease or malnutrition, can prevent a person from reaching his or her full genetic height potential.

"We know environment can affect heights by three, four, five inches," said Richard H. Steckel, an Ohio State University economist who has also done research on height trends in the United States during the 19th century.

The earliest stages of life are the most important to the human growth machine; at age 2 there is already about a 70 percent correlation between a child's height and his or her eventual adult stature.

All of this means a population's average height is a very sensitive indicator of its most vulnerable members' welfare.

Not surprisingly, rich countries tend to be taller simply because they have more resources to spend on feeding and caring for their children. But wealth doesn't necessarily guarantee that a society will give its children what they need to thrive.

In the Czech Republic, per capita income is barely half of what it is in the United States. Even so, Czechs are taller than Americans. So are Belgians, who collect 84 percent as much income as Americans.