If your parents are nearsighted (or short-sighted, if they happen to be British), you’re likely to be nearsighted too. The heritability of this problem is as high as 90% in some twin studies. But that’s only true if you and your parents or siblings grow up in a similar environment.
When you shift to a different lifestyle, the outcomes change. In 1974, a team of researchers evaluated a community of Inuit in the Canadian Arctic soon after formal schooling was introduced. Of adults over 30, only a small fraction (4.5%) were nearsighted. In contrast, 45% of younger adults, ages 15-29, were nearsighted – a tenfold rate increase, which happened much faster than genes in the population could vary.
The increase in nearsightedness that struck the Inuit in one generation has spread all over the world. The US rate went from 25% in the early 1970s to 42% three decades later. In a few societies, nearsightedness rates are now nearly as high as they can get. Among modern Chinese students in Singapore, 90-95% are nearsighted.
Research in lab animals and epidemiology in people suggest that the culprit is too much time spent indoors. Weak artificial light does not provide the signals that the developing eye needs to grow to the right size.
Do these rapid changes mean that genes don’t matter to nearsightedness? Not at all. Children with a genetic predisposition are more likely to become nearsighted – or more severely nearsighted – than children whose parents have good vision.
Here’s one way to think about it. Picture 100 children lining up to receive their portion of nearsightedness. In an environment where kids spend all day outside, there’s only a small amount available, enough for the first three or four children in line. The rest get normal vision. In Singapore, there is a huge amount of nearsightedness, so the first few kids get a lot more, and there’s still enough left for almost everyone to get some.
In both cases, the kids line up in the same order, which is determined by their genetics. Regardless of the environment, the children at the front of the line (the most vulnerable) do worst, and those at the back of the line do best. But the amount of outdoor time determines how far down the line the problem is distributed.
Obesity has also increased in the population much faster than genes could change. Weight is as heritable as height. And like nearsightedness, weight is influenced by the environment, with the strongest effects on the genetically vulnerable.
Scientists don’t yet know which environmental changes are most important for the increase in obesity. Candidates include processed foods, antibiotics, stress, sleep loss, prescription drugs, reduced exercise, and decreased cigarette smoking, among others. Whatever has changed over the past 50 years, it probably isn’t our collective willpower.
So why do we treat nearsighted and obese people so differently? You wouldn’t tell a person who wears glasses, “Well, it’s your own fault. You should have played outside more as a child.” But even health care professionals tend to blame their patients for being overweight, though they should know how powerfully the body fights against weight loss.
Maybe it’s time to rethink our cultural focus on individuals and their willpower as the cause of weight gain. In addition to being more fair, concentrating on the bigger picture might give us some ideas that actually work to promote health and fitness.