Ask A Scientist
Why does your body grow fast as a teenager?
Asked by: Addison Duff
School: St. John the Evangelist
Teacher: Anu Rai
Hobbies/Interests: Reading, drama
Answer from Michael Little
Distinguished Professor of Anthropology
Research area: Human adaptation to the environment Interests/hobbies: Swimming, choral singing, antique toys, books
The human body normally grows in a very specific pattern. Babies grow very fast during their first year, and then slow down in childhood. Between the ages of 2 and 9 or 10 years, children grow very slowly, but steadily. When girls reach about 10 years of age, they begin growing rapidly, and also begin to sexually mature and be transformed into women. Boys begin growing rapidly about two years later than girls, at about age 12. This period of rapid growth is called adolescence and lasts about three years in both boys and girls. When children begin this growth spurt, as it is called, the beginning period of rapid growth is called puberty (the period when maturation begins).
The ages that I have given here are just averages for girls and boys in the United States. There is lots of variation in when children reach adolescence and how fast they grow during adolescence. If you look at sixth and seventh graders, some are tall and have already started adolescence, while others are short and haven't yet begun the growth spurt. Some of these differences were inherited from parents, while others result from the way lives are led. For example, when children are poor (in the U.S. or in other countries), they may not have enough food, which slows down their growth and may even prevent them from reaching a normal adult size. There are lots of ways that the growth of children and adolescents may be affected. As I mentioned, not enough good food in the diet can slow growth, and various illnesses may have the same effect. Children in poor countries may not only grow slowly, but they may have a very late adolescence (to begin at 16 or 17 years of age). When children in poor tropical countries have diseases like malaria or other serious infections, their growth patterns will not be normal and healthy.
We know how children and adolescents grow and reach the size and maturity of adults. And we know what can change this pattern of growth (a healthy life or a poor life). But we do not know exactly why this pattern exists for humans. For example, babies increase their birth weight three times by their first birthday. They are very fat and healthy at one year of age, and this rapid growth helps them to survive. When adolescents grow rapidly, it may help them to become adults (with an adult size) quickly to avoid some difficulty with other members of their society, but this just a guess. We also do not know why girls mature earlier than boys, but we have some guesses for this also. We do know that our closest relatives, the chimpanzees, do not have an adolescent growth spurt, so adolescence is just one of the human characteristics that must be a part of our evolutionary history.
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