Don't Be Myopic About Heritability
According to a news feature from the journal Nature, shortsightedness could be on the rise because children are spending less time outdoors than they used to.
In short: myopia has undergone a marked increase in the past 50 years, and several studies link this rise to the amount of time that children are spending indoors and away from high levels of light, which could play a role in regulating eye development. An Australian researcher cited in the story, Ian Morgan, estimates that children need at least three hours of strong light per day to be protected against this vulnerability.
The so-called "myopia boom" provides a nice illustration of the complexities that arise in attempting to parcel out environmental versus genetic influences on biological traits. One way to quantify how much variation in a trait — such as shortsightedness — is due to variation in genetic factors is in terms of the trait's "heritability," a technical notion that (in its narrow sense) is defined as the "proportion of the phenotypic variance that can be attributed to additive genetic variance."
To estimate the heritability of a trait, one approach is to measure how frequently that trait is shared by identical versus fraternal twins, on the assumption that observed differences are due mainly to the extra genes shared by the former set. According to the article at Nature, studies of this kind conducted in the 1960s suggested that susceptibility to shortsightedness "is strongly influenced by DNA."
But here's where things get tricky, and why the myopia boom is such a nice example of what heritability does — and doesn't — mean. When the environment changes, as it has for today's indoor-dwelling children, so can a trait's heritability.
Consider two generations of children. In the first generation, virtually all children spend long hours outdoors. For this group, differences in exposure to light are unlikely to account for much variation in shortsightedness: Almost no children will fall below the rough three-hours-of-light-per-day minimum that Morgan suggests. And with this source of environmental variation in the trait reduced, genetic influences could play a large role. But fast-forward to our second generation of children and you could have a population whose members vary quite dramatically in exposure to light. Studies in this group could reveal considerably lower estimates of heritability than those from the first generation, simply because the environment has changed and now accounts for more of the variation in the trait.
The complications don't end there. As environments change, so can which genes are associated with particular traits. Suppose, for example, that some genes influence precisely how much light is required to regulate eye growth. These genes could become strongly predictive of myopia in our second generation — many of whose members receive marginal amounts of light — despite having little correlation with myopia in the first generation.
If this seems more than a little counterintuitive, it could be because heritability is often confused with related concepts, such as innateness and genetic determination. And this confusion could fuel misconceptions, like the idea that highly heritable traits aren't susceptible to environmental influence, or that a trait's heritability is constant across populations and over time. In fact, a trait with high heritability could become highly susceptible to environmental influence as environments change, and a trait once governed by environmental variation could become more heritable if the range of relevant environments becomes constricted.
So, when it comes to thinking about the factors that give rise to biological traits, it might help to be farsighted — the better to see the possibilities beyond the populations and environments that happen to be right before our eyes.
Tania Lombrozo is a psychology professor at the University of California, Berkeley. She writes about psychology, cognitive science and philosophy, with occasional forays into parenting and veganism. You can keep up with more of what she is thinking on Twitter: @TaniaLombrozo.
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