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What makes us as tall as we are?

My daughter doesn’t like being at the short end of the grade three height spectrum. She doesn’t accept the “good things come in small packages” argument or “the bigger they are, the harder they fall” scenario. I decided to see what science had to say about why people are the size they are.

Tall Tales

Taller people apparently make more money, live longer, and are seen as more attractive, among other things. Or maybe being rich, lets you grow taller. US presidents seem big on height, but the trend is not so clear in other places. Taller people I know complain about finding clothes, walking through doorways and sitting on airplanes. On the other hand, shorter women tend to have more children, which might be of evolutionary significance. Or maybe having more children limits their growth. In any case, I don’t think my daughter will find that result particularly satisfying.

All in the Family

And you can’t really do much about how tall you are. Genetics accounts for 60 to 80 per cent of height, although these studies are often seem to be based on comparisons with relatives, so I’m not sure how well they tease out the environmental factors. American white males are in the 80 per cent range. But studies in China and Africa are in the 65 per cent range. In North America, nutrition is not usually a limiting factor. So they are suggesting that in developing regions, nutrition tends to be the limiting factor. Younger generations in general seem to be getting taller and this is attributed to better nutrition, although junk food can lead to subnutrition.

Calculating the Future

Even if you eat an ideal diet from birth, about 180 sites of genetic variation have been found in humans related to height. You can find online calculators to predict the adult height of a child, if you know the measurements of the child and his or her biological parents. They say the result will be within about 10 cm but with that much leeway, you could probably just guess.

Ys, Men

On average, men are taller than women. Two genes have been found that account for up to 4 cm of a man’s height. One is on the y chromosome of males, which tends to be a haven for gender differences. One of them is on chromosome 15, and codes for aromatase. This enzyme converts testosterone into estrogen. Estrogen is a busy hormone. In addition to all the other female things it does, it also leads to the fusion of the ends of bones, which stops growth. The main difference in height between men and women is in the length of the legs. I guess this means few adjustments in the construction of all the internal organs.

This morning, when I suggested my daughter drink her milk if she wanted to grow taller, she just gave me the “Whatever”. I’m going to take that as a sign she has achieved more self-acceptance.