"Bookworms more likely to end up shortsighted," reports the Daily Telegraph after a study found people who stay in education longer are more likely to develop short-sightedness.
It's long been known that there is a link between short-sightedness and the length of time spent in education.
But researchers have now used a genetic technique to suggest that spending longer in education may cause short-sightedness, rather than short-sightedness leading someone to stay in education longer.
Researchers used genetic profiles, sight tests and education records of 67,798 adults who took part in the UK Biobank study. They analysed whether people with genetic variants linked to short-sightedness tended to stay in education for longer and whether people with genetic variants linked to more time in education tended to be more short-sighted.
The results showed that people with a genetic tendency to spend longer in education were more likely to develop short-sightedness. But the reverse was not found: a genetic tendency for short-sightedness did not seem to affect the number of years in education.
The results don't tell us why education might affect vision or what, if anything, could be done to prevent people developing short-sightedness. However, other studies have suggested that spending time outdoors may be beneficial.
The study was carried out by researchers from the University of Bristol and Cardiff University. They received funding from the Medical Research Council, Bristol Centre for Systems Biomedicine, National Eye Research Centre and National Institute for Health Research, and from the Russian government's Global Education Program.
It was published in the peer-reviewed British Medical Journal on an open-access basis so is free to read online.
The Daily Telegraph's headline was a little misleading – the study found that short-sightedness was caused by spending longer in education, not by being a "bookworm".
BBC News had an amusing and informative article on the study, explaining the science through an imagined question-and-answer exchange between a child and teacher.
This study combined a traditional cohort study with a technique called Mendelian randomisation.
Mendelian randomisation uses the fact that genetic variants are inherited independently of possible confounding factors – which can otherwise affect outcomes – to make it easier to see whether 2 factors are linked by cause and effect.
By using genetic variants known to be linked to traits such as short-sightedness and length of time in education, the researchers were able to reduce the bias caused by potential confounding factors, such as socioeconomic status.
Researchers used anonymised data from men and women taking part in the long-running UK Biobank study of genetics, lifestyle and health.
People in the study gave DNA samples and filled in questionnaires, including information about their education, and some also took tests for short-sightedness.
After excluding people with eye conditions such as cataracts or those who had undergone laser eye surgery, the researchers analysed the data of 67,798 participants.
They used 2 previous studies that had identified genetic variants associated with short-sightedness and with the number of years of education. These studies identified 50 variants associated with short-sightedness (44 of which could be used in the present study) and 74 associated with years in education (69 of which could be used).
The researchers first carried out a standard cohort study looking at associations between years in education and short-sightedness, adjusting for deprivation, birthweight, breastfeeding and where people were born.
They then carried out 2 Mendelian randomisation analyses using:
They also did sensitivity analyses to check for the effect of potential confounding factors, and for the possibility that some genetic variations may affect both time in education and short-sightedness.
Short-sightedness was assessed in dioptres, a unit of measurement of the eye's ability to focus light onto the retina. A dioptre of -1 is enough to need glasses for driving.
As expected, the standard cohort study analysis showed people who spent more time in education had, on average, a greater degree of short-sightedness (-0.178 dioptres for each additional year in education, 95% confidence interval [CI] -0.185 to -0.170).
Using Mendelian randomisation, the researchers found:
The sensitivity analyses showed few signs that other factors could have affected the results.
According to the researchers, the study "provides strong evidence that more time spent in education is a causal risk factor for myopia [short-sightedness]", and given the rapid spread of short-sightedness, especially in China and east Asia, "the findings of this study have important implications for educational practices".
They said: "Policymakers should be aware that the educational practices used to educate children and to promote personal and economic health may have the unintended consequence of causing increasing levels of myopia and later visual disability.
"The best recommendation based on the highest quality available evidence at the moment, is for children to spend more time outside."
This study adds some fairly convincing evidence to the theory that more time in the school room, at an age when eyes are developing, has a potentially damaging effect on children's eyes. However, it's important that children don't stop going to school, and that the rise in literacy and spread of education around the world continues.
So what can be done to encourage children's education while protecting their eyesight?
This study cannot answer that question because it cannot tell us what it is about time spent in education that seems to cause short-sightedness. But as the authors said, ensuring children also spend time outdoors, where they can get plenty of bright daylight and use their eyes over long distances, seems to be the best bet at present.
The study was well conducted but did have some limitations.
Firstly, genetic variation was only weakly linked to the outcomes – the researchers estimated that genes accounted for only 4.32% of the difference in short-sightedness and 0.71% of the time spent in education.
It found some evidence that geographical factors, including how far north people lived, also affected short-sightedness.
Finally, participants in UK Biobank tended to be more highly educated and have healthier lifestyles than the general UK population, so the results may not apply to everyone.
Read more about short-sightedness, how to test for it and what can be done about it.