Most of us attribute our height to genetics. I fault my dad for my short stature, who swears he’s 5' 10" (subtract an inch or so). My brothers hit the genetic jackpot and are tall like my mom, who claims to be 5' 11" (and is actually closer to 6 feet). But while my parents’ genes and random chance explain my height compared to my siblings, it doesn’t account for height variations between whole populations, like why American women are on average nearly two inches shorter than Dutch women are. Heredity may even play less of a role in the dramatic differences we see between say, the long, lanky Kenyan Masai and petite Filipinos. Researchers are finding that equal, if not more influential, predictors of height in the modern world are environmental factors including nutrition, exposure to disease, and the quality of health care.
The Climate Effect
Two basic rules have dominated science’s understanding of height since the 19th century. They simply documented what the eye could see. Taken together, Allen and Bergmann’s rules tell us that warm-blooded animals in very cold climates tend to be more compact, with short appendages. Their bodies have a comparatively small surface area, helping them retain warmth—think polar bears. Closer to the equator, animals tend to be lean and tall with long limbs, encouraging heat to escape and cool their bodies—think giraffes. In human terms, the classic illustration of these principals is the oft-cited comparison of the short, stocky Inuits living in subarctic temperatures and the long-limbed Masai and Dinka tribes near the equator.
Recent research has provided some fodder for these rules, not as proof of natural selection, but of proof that climate can influence physiological development. At the University of Pennsylvania, researchers separated mice into warm and cold environments. The mice in warm temperatures grew longer ears, tails, and limbs than the cold-reared mice did. Researchers suggested that the cold restricts blood flow, stemming cartilage growth.
Even so, the University of Pennsylvania study fails to explain a lot about human height in the past and present. Why, for instance, were the Cheyenne, who inhabited the Great Plains, the tallest people in the world in the late 19th century? The Midwest gets muggy in the summer, but also has harsh winters. Or why do Guatemalan women living in hot, humid climates rarely reach five feet? And perhaps the biggest question is for the United States. Why, if cold inhibits growth, are the Dutch, who a century ago ranked among the shortest people in the world, are now the tallest—a good two to three inches above Americans?
