Isolating the causes behind one hundred years of increasing human heights
At dinner each night we often have to remind our kids to eat their “grow food.” It’s basically a catch-all term for food that has more nutritional value than a french fry, and is meant to remind them of the promise that if they eat a balanced diet, they may grow to be bigger than their parents. This is only partially true, and my second grader is definitely becoming aware of a few caveats to this concept. She’ll probably be taller than her mom, but sexual dimorphism in humans make it unlikely she’ll be taller than me. It turns out that while a good diet and genes for height definitely help kids grow taller, these factors don’t explain how common it’s become for kids to end up bigger than their parents. It’s not just an abundance of “grow food” that’s lead to the last 100 years yielding taller and taller humans.
More resources in reach
The height gains seen in Homo sapiens since World War I is most clearly demonstrated in data from western countries’ data on their enlisted soldiers. While not a perfectly diverse sample, the number of people included in the data convincingly shows a gain in average height of around four inches. It’s a big jump in a short time, and since it’s safe to assume that these soldiers didn’t all experience matching mutations in their DNA, scientists have been looking to environmental factors to explain why people have been getting bigger.
Some of the obvious culprits would be that people’s standards of living have been improving, and that more, better food has been available at the same time. More “grow food” being eaten by kids doesn’t hold up entirely though, because the increase in height hasn’t held up perfectly either. Looking back further, at the beginning of the 20th century many countries saw more food becoming available, and per capita incomes going up as well. However, average heights of humans actually fell in these countries at the same time, with the benefits of industrialization not helping people grow alongside new resources.
Spending energy on size, not sickness
The answer is then the other side of health, which is exposure to pollutants and disease. Great nutrition can’t fuel growth if a growing body is using all its energy to fight respiratory or gastro-intestinal infections. So people growing up in eras of extreme pollution or disease outbreak broke the trend of growing taller, revealing what the last factor in what can help a kid grow up to reach their full potential.
These trends are actually visible even before the advent of disease-preventing vaccines, meaning that humans have been figuring out ways to reduce our health risks for generations. Improvements in sewage treatment, overcrowded housing and generally lower exposure to billowing smokestacks all helped humans grow taller, without any specific medical intervention. Smaller families since the Victorian era meant more resources would be devoted to the health of each child. In the 20th century, one of the biggest boosts to a kids’ growth has been their parents’, and especially their mother’s, increasing education and understanding of health and hygiene. All these factors basically work to remove the brakes on our genes’ potential for being a bigger species. Especially if we eat our broccoli.
Source: Why did humans grow four inches in 100 years? It wasn’t just diet by Tim Hatton, The Conversation
