Why Have Americans Stopped Growing Taller?

July 3, 2001 -- There was a time when most American children were expected to outgrow their parents.

Since data on Americans' average height was first collected in the early 20th century, children and adolescents grew about an inch and a half taller every 20 years. But recent measurements suggest Americans' average height has more or less hit the ceiling.

Data collected from the federal Centers for Disease Control show that average height for Americans has stabilized in the past 50 years to about 5 feet 9 inches for men and 5 feet 4 inches for women.

"We've pretty well maxed out in terms of stature. There's been little change in adult height over last generation," says William Leonard, an anthropologist at Northwestern University.

You Are What You Eat

The reason, explains Leonard, is most Americans now face few nutritional or health-related stresses in their youth. People grow most as infants and then as adolescents, and most Americans have avoided disease and eaten enough meat and milk in their youth to reach their genetic height potentials.

But Leonard is quick to point out that while overall U.S. height averages have more or less stabilized, there are small pockets of the population where slight increases in height are likely still happening (small enough not to greatly affect overall averages).

Zero in on immigrant populations, he says, and on communities where socio-economic constraints lead to malnutrition and health-care problems, and generations of new children continue to grow taller since height maximums have not yet been reached.

"There is large heterogeneity in ethic compositions," he says. "When you look at Asian-Americans, Hispanic Americans — those are the pockets where increases in height are still happening."

Studies dating from the 1930s have demonstrated how a person's environment — and in turn nutrition — can directly affect not only a person's size, but also dimensions.

In analyses done on European immigrants to the United States and their children 60 years ago, researchers showed immigrant children born in the United States were taller and had larger heads and broader facial features than their foreign-born parents and siblings who were born abroad.

More recent studies, done in the 1960s and 1970s have indicated the importance of nutrition to height even more directly. Over this time period anthropologists analyzed a series of villages in Central America. To infants in some villages, they provided nutritional supplements, while in others they supplied only placebos.

Overwhelmingly, the children who had received the supplements in youth grew taller and were even more successful throughout life.

Growing Seasons of Life

When a person receives good nutrition is critical since growth occurs almost exclusively in infancy and adolescence. Humans grow fastest during the first two to three years of life, then growing slows through childhood until about age 10 or 11.

At this point, says Leonard, girls accelerate in height to reach their maximum by age 17 while boys shoot up a little later, reaching their tallest by age 20. One of America's most famous tall people, Wilt Chamberlain, hit a major growth spurt at age 15, when he grew four inches in three months. By the time he started playing college basketball he had reached a towering 7 feet 1 inch.

Growth beyond the age of 20, says Leonard, is rare. But interestingly, late growth spurts are more likely to occur when poor nutrition has inhibited growth earlier in life.

In the malnourished communities in South America, for example, people continued growing throughout their 20s. But these people still remained shorter than most Americans, since they grew less during the more critical years of childhood.

Since tallness reflects a person's early health and nutrition, tallness, in turn, can be a good indicator of a person's longevity. A recent study by David Gunnell, an epidemiologist at England's University of Bristol, showed that at least in older times, good height meant a long life.

Height Linked to Longevity

Gunnell measured hundreds of human arm and leg bones exhumed from English graves from between the ninth and 19th centuries. His studies showed that the longer the bone length (bone length correlates to overall height), the more likely a person lived beyond the age of 30.

In modern times being tall can help a person be more successful — sexually and professionally. In the United States studies have shown taller presidential candidates are more likely to be elected (with the notable recent exception of George W. Bush) and anecdotal evidence suggests that taller people appear more attractive and so have an easier time finding sexual partners.

When Height Is a Hindrance

But Wolf Blanckenhorn of the University of Zurich, Switzerland, points out that animal evidence show that being too tall can have drawbacks.

Oversized animals need to eat more and so can have a harder time surviving when food is scarce. Taller animals also tend to be less agile and so are more vulnerable to predators and parasites.

In people, being very tall can add strain to the circulatory and skeletal systems. And anecdotal evidence points to other difficulties.

"Think of all the little disadvantages tall basketball players have in everyday life with food, clothing, living quarters, beds, etc.," Blanckenhorn points out.

For these reasons, anthropologists think it's unlikely that Americans will develop a new genetic pool to become even taller, now that they appear to have reached their genetic growth potentials.

That means that very tall Americans are likely to continue to be perceived as, well, being very tall. And, as Leonard points out, being different in any way — even in height — isn't always easy.

Chamberlain may have won fame and adoration for his height and skill on the court, but he had to have special fixtures put in his house to accommodate his size, and was known to say:

"Nobody roots for Goliath."