What's more important, genes or environment? It's an argument that goes back to well before the concept of genes was developed. The advent of genomics has given us a new perspective on the role of genetic variants in a lot of traits, but it hasn't necessarily stopped the arguing. The reasons are nicely highlighted by a new paper that describes a massive genetic screen for factors associated with educational success.

The study screened more than a million people for genetic variants associated with time spent in school. And while the study came up with a lot of genomic regions that were associated with schooling, the average difference made by each individual gene is only 1.7 weeks; collectively, the regions only account for a bit more than 10 percent of the differences in time spent studying. Critically, they may only be relevant to the European populations that were used to identify them.

Find the genes

Time spent in school is a rough measure of educational achievement. While there may be radical differences between two people who completed college, there's likely to be some consistent differences between people who quit after high school and those who pursued graduate study. And it seems to be a relevant measure in that it correlates with the improved economic and health outcomes that are typically associated with successful careers. And it's a less biased way of getting at educational achievement than things like standardized tests.

For all these reasons, there have been previous attempts to identify which parts of the genome are associated with time spent in school. One study surveyed the DNA of more than a quarter-million people to identify 74 distinct regions of the genome associated with educational endurance. But the new work goes well beyond by involving 1.1 million subjects.

The additional subjects provide a very different perspective, identifying 1,271 sites in the genome that appear to be associated with time spent in school. The researchers then looked at genes located near these sites and found that a large majority of them were active in the nervous system—during its development, in adulthood, or both. The genes affected nearly every aspect of nerve function, like allowing ions to move across cell membranes, nerve cell metabolism, forming and maintaining connections among nerve cells, and more.

One category of genes was strikingly absent, though: those expressed in glial cells, which support and maintain the nerve cells. Glial cells, among other things, build sheaths of a fatty material called myelin, which acts as an insulator to isolate the electrical activity of nerve impulses. It had been suggested that some aspects of cognition were dependent upon the speed and efficiency of nerve impulses, which in turn depends on this insulation. So the study raises questions about that idea.

Big and small

All of this information suggests that the basic approach is on to something. But the "something" turns out to be either dramatic or not all that interesting, depending on how you look at it. With nearly 1,300 genes involved, the differences between people who end up at one or another of the extremes can be dramatic. Someone with a lot of variants predisposing them toward staying in school is far more likely to do so than someone with very few.

But many people don't lie at either of these extremes. And when it comes down to individual genomic sites, the effects are small; even the strongest ones only add an average of 2.6 weeks of extra time in school (it's 1.1 for many of the weaker ones). And the entire set of 1,271 sites can only account for about 12 percent of the total variation in educational attainment, meaning nearly 90 percent of the variation is caused by something else. We do know a couple of those causes—these genes edge out household income in terms of the strength of their impact but come in behind having a parent with an extended educational background.

Put differently, if your mom has a PhD, it will probably have a stronger impact on your educational trajectory than your genes will.

The authors themselves suggest that these genomic sites aren't necessarily useful for studying education in and of themselves. Instead, they should be used as a control when studying the factors that influence the other 90 percent. If you can account for genetic influences, then environmental factors should stand out more readily.

At least among some populations. While the study was done in the US, the researchers limited the experimental population to people of European descent, since that would make it easier to ensure a large number of participants. The researchers were also looking at African-Americans, however, and testing these same markers on them showed that they were only weakly predictive of educational attainment. There are plenty of possible explanations for this, from African-Americans having an equivalent-but-distinct set of factors to the fact that social factors have historically dominated their access to education. At the moment, though, we simply don't know enough to say.

While we may learn more by examining other human populations, the pieces of genome identified here are probably pushing the limit of what we're going to discover. Pushing the number of people screened far beyond one million will likely only allow us to identify factors with even weaker effects, and the 1,271 sites we've identified already account for nearly five percent of all human genes (assuming one gene per site). So any significant advances from here on out will likely rely on trying to figure out how these genes might interact with environmental factors.

Nature Genetics, 2018. DOI: 10.1038/s41588-018-0147-3 (About DOIs).