Every Friday this month we’ll be taking a look at the relationship between the social and the biological—specifically, how and why the former becomes the latter. Check back each week for a new installment.
It’s one of the most irresistible and controversial questions in science: "How and why does the social become biological?" The classic mind-body split is something we feel intuitively, a distinction between our "biology"—our anatomy or our physical health—and our social, decision-making minds. So it can be jarring to hear claims like this: "People differ in their intelligence, personality, and behavior, and a century of research in behavioral genetics leaves little doubt that some of this variation is caused by differences in their genomes." No matter where we look, we always find the influence of genetics—the social is never entirely free of the biological. But what does it really mean to say that social or mental traits are influenced by genes?
Consider two traits that didn't evolve "naturally," but rather were completely invented by humans: the ability to read and do mathematics. Last month, a consortium of scientists sponsored by the Wellcome Trust published a study showing that "the correlation between reading and mathematics ability at age twelve has a substantial genetic component." In other words, reading and math abilities aren't merely the result of a decision to work hard at them, or an opportunity to learn; they also depend on DNA.
Genetic distinctions affect every chemical process in our cells, and because absolutely nothing we do happens without some cell chemistry, everything about us is potentially influenced by genetic variation.
In one sense, this is trivially obvious. The reason that children learn to read and chimpanzees don't has nothing to do with the chimps' lack of educational opportunities; it's entirely genetic. The physical nature of our brains allows us to develop mental skills that are hopelessly out of reach for other animals.
But when we say a mental trait is influenced by genetics, we clearly mean more than that; we're also making a statement about the role of genetic differences among people. Learning to read and do math involves some very complex brain biology. New links are made between different specialized areas of the brain, and old parts are re-purposed to engage in something that human brains didn't do until relatively recently. All of this brain rewiring depends intimately on the chemistry of our neural cells, chemistry that is subtly altered by thousands of genetic differences that change the properties of the molecules involved. Unlike the precision engineering that goes into the latest Intel chip, the human brain's process tolerances are rather wide—nearly all of our molecular parts show some variation among the human population.
Genetic variation is an unavoidable and central fact of biology, and it is at the heart of the relationship between the social and the biological. There never was a master copy of the human genome; species nearly always exist as populations of genetically varied individuals. These genetic distinctions affect every chemical process in our cells, and because absolutely nothing we do happens without some cell chemistry, everything about us is potentially influenced by genetic variation.
The Wellcome Trust researchers studied the influence of genetic variation on math and reading skills, and the correlation between them. Using data collected from nearly 3,000 sets of twins who had taken standard reading and math tests, they used two methods to examine the role of genetics.
In the first method, they searched directly for an association between a specific DNA difference and scores on tests for math or reading. The idea behind this is fairly simple: Consider a place in the human genome where people differ—some people may have the chemical letter "A," while others have "G." Do people with an "A" have, on average, higher test scores than those with a "G?" You repeat this test for thousands or millions of different places in the genome and find the DNA differences with the strongest association. In practice, scientists don't merely compare the averages of "A" versus "G"; they use a more powerful statistical procedure, but the point is the same: to find specific genetic variants that correlate with differences in test scores.
The second approach, using the resemblance between twins, is more indirect—it doesn't involve knowing any of the actual DNA differences involved. Since twins share a common family environment, but identical twins are closer genetically than fraternal twins, it's possible to quantify the genetic and environmental influences on a trait without saying specifically what those influences are.
From these analyses, the researchers came away with three big results that nicely illustrate what it means to say that a behavioral or mental trait has a genetic component. First, the twin analysis showed that genetics explains many of the differences in reading and math test scores in the studied population. But the researchers were unable to find any reproducible association between any specific DNA difference and reading and math ability, suggesting that the total genetic effect is the cumulative result of small changes in many different genes. And finally, they found that not only were reading and math scores influenced by genetics, but also that the correlation between reading and math scores showed a strong genetic influence, suggesting that these skills are influenced by “generalist genes.”
What is a "generalist gene"? The idea, proposed years ago by Robert Plomin, one of the study authors, and Yulia Kovas, is that many of the genes involved in cognitive processes don't have highly specialized roles limited to one part of the brain. Instead, each generalist gene influences many different brain processes, and conversely, each brain process is the combined result of many different genes. Hence, common genetic variation in any one gene will have only a small effect, but on many different traits at the same time.
Importantly, genetic studies like this one also say something about the importance of the environment. The authors argue that "our results highlight the potential role of the learning environment in contributing to differences in a child’s cognitive abilities at age twelve." They're suggesting that when a child's reading and math abilities—which should be correlated—diverge from each other, there is an opportunity to make a productive change in the learning environment.
Genetic variation influences every cellular process, and everything we do depends in some way on the processes in our cells; ultimately, the social and the biological are inseparable.