When we say "it's genetic," we're suggesting that "it" — a disease, a physical trait, or someone's IQ — is something innate that can't be changed. We think of genetics as a fixed part of ourselves that is distinct from the more malleable influences that shape who we are, such as our culture, our lifestyle choices, and our physical environment. The sharp distinction we draw between the genetic factors we can't control and the social and environmental ones that we can, strongly influences how we see ourselves. It also sets the terms of debate for many contentious social issues: Is our intelligence fixed? Can we change our sexuality or are we just born with it? How much does our health depend on the choices we make?
But by splitting biological and social aspects into separate categories and resigning ourselves to the inevitability of one of them, we're mis-framing these questions. It is true that we can't change our DNA (at least not yet). The real story, however, is about how our genes and our environment interact, which is something that geneticists have long known, but which rarely makes it into public discussions about genetics. It's important to get this story straight, because modern genetic studies are becoming part of the discussion of not just diseases like cancer, but also social issues like income, education, and even political beliefs. We're bound to misinterpret these studies if we fail to recognize how thoroughly our genes and our physical and social environment are tied up together.
Two recent genetic studies illustrate the very conditional role that our genes often play. In one study, researchers looked at how smoking, alcohol consumption, and genetics alter one's risk for colon cancer. Analyzing two large study cohorts of about 20,000 subjects each, half of whom had colon cancer, the researchers first found what other studies have reported — that smoking and heavy drinking generally increase your risk of developing colon cancer, period. They also found, as some other studies have reported, that light to moderate drinkers have a slightly lower risk of colon cancer, relative to those who never drink.
But, interestingly, the effect of light to moderate drinking depends on genetics. The researchers found that light drinkers with a particular genetic variant in the gene HIATL1 did not see their risk of colon cancer go down relative to non-drinkers. The beneficial effect of moderate drinking was limited only to those with a different variant in HIATL1. How exactly this works isn't clear; HIATL1 is a gene that is expressed in colon cancer tissue, but its role is not known. The researchers did not find an interaction between genetics and smoking, but that doesn't necessarily mean one doesn't exist — these interactions are often subtle and difficult to detect.
In another study, another group of researchers examined the influence of lifestyle and environment on the role of a gene that has been linked with obesity. Genetic variants near a gene called FTO are associated with a small but clear increase in body mass index (BMI), a standard measure of obesity. People who carry one or two copies of the "A" genetic variant tend to have a BMI that is 0.4 to 0.8 units higher than those who have two copies of the "T" variant. (For a sense of scale, a BMI of 25 or higher is considered obese — so the genetic effect is relatively small.)
The researchers looked at several dozen lifestyle factors, including sleep, exercise, diet, smoking, and drinking, in three large United Kingdom study populations, and asked whether any of these factors changed the impact of the high-BMI genetic variant. Perhaps not surprisingly, the factor with the strongest effect was exercise: People who carry the high-BMI variant, but who exercise vigorously, cut the genetic effect of that variant by half. Interestingly, the researchers also found that light to moderate drinking reduced the genetic effect of the high-BMI variant, which is consistent with earlier studies that reported a link between light drinking and a lower BMI.
These two studies — whose results should be considered provisional until replicated more broadly — are focused on medically relevant traits like cancer and obesity. But the lessons here extend to genetic studies that look at important social traits, like educational success, cognitive ability, income, and aggressive behavior. In all of these, there are without a doubt an enormous number of interactions between genetic factors and social ones. Such interactions can be difficult to detect statistically, and so most genetic studies published today do not report on them. But don't be fooled — they do exist, and they are important.
Each of us has millions of genetic variants — places in our DNA where we differ from other people — that play complex roles in traits ranging from our skin color to those aspects of our personality that drive our political beliefs. As genetic studies grow larger and DNA technology improves, we are likely to find that genetics makes at least some contribution to almost everything about us. But the converse is also true: The impact of nearly every genetic factor is affected by our physical and social environment. The reason for this is no mystery — the very process of being alive requires genes to respond to signals from the outside, including signals from other people.
This means that we can't neatly divide ourselves into social and biological components, especially when it comes to addressing some of the most salient social issues today. Sexuality, for example, can't be reduced to a debate over whether it’s innate or not — and the effort to defend the rights of LGBTQ people should not come down to an argument over whether or not their identities are a choice. For intelligence, another frequent subject of misguided debates, genes matter — but acknowledging the role of genes doesn't mean we can then dismiss the enormous impact of our social environment.
The deep connection between genes and environment is an old truth in genetics, but it's one that's easy to forget when we see headlines about the latest genetic study. If we forget it, we risk misunderstanding ourselves.
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