Why Have Americans Stopped Growing Taller?

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.

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